shouldn't add ssl_client_cert and ssl_client_cert_key if no tls.crt and tls.key in kafka secret

Description of problem:
For Server-authenticated only TLS, if the Kakfa cluster is using self-assigned certificate.  I create the secret using only the ca-bundle.crt 'oc create secret generic kafka --from-file=ca-bundle.crt=ca.crt'.  

Three ssl variables into fluentd.conf, the fluentd couldn't be started as no tls.crt and tls.key.   The CLO should add ssl_ca_cert only.
 ssl_ca_cert '/var/run/ocp-collector/secrets/kafka/ca-bundle.crt'
 ssl_client_cert '/var/run/ocp-collector/secrets/kafka/tls.crt'
 ssl_client_cert_key '/var/run/ocp-collector/secrets/kafka/tls.key'


Version-Release number of selected component (if applicable):
4.6

How reproducible:
Always

Steps to Reproduce:
Always

1.oc create secret generic kafka --from-file=ca-bundle.crt=ca.crt 
2.oc create ClusterLogForwarder CR
apiVersion: logging.openshift.io/v1
kind: ClusterLogForwarder
metadata:
  name: instance
  namespace: openshift-logging
spec:
  outputs:
    - name: kafka
      type: kafka
      url: tls://my-cluster-kafka-bootstrap.openshift-operators.svc.cluster.local:9093/bridge-quickstart-topic
      secret: 
        name: kafka
  pipelines:
    - name: test-app
      inputRefs:
      - application
      outputRefs:
      - kafka

3. Check the fluentd.conf
 <label @KAFKA>
      <match **>
        @type kafka2
        brokers my-cluster-kafka-bootstrap.openshift-operators.svc.cluster.local:9093
        default_topic bridge-quickstart-topic
        ssl_ca_cert '/var/run/ocp-collector/secrets/kafka/ca-bundle.crt'
        ssl_client_cert '/var/run/ocp-collector/secrets/kafka/tls.crt'
        ssl_client_cert_key '/var/run/ocp-collector/secrets/kafka/tls.key'
        <format>
          @type json
        </format>
        <buffer bridge-quickstart-topic>
          @type file
          path '/var/lib/fluentd/kafka'
          flush_interval 1s
          flush_thread_count 2
          flush_at_shutdown false
          retry_max_interval 300
          retry_forever true
          queued_chunks_limit_size "#{ENV['BUFFER_QUEUE_LIMIT'] || '32' }"
          chunk_limit_size "#{ENV['BUFFER_SIZE_LIMIT'] || '8m' }"
          overflow_action "#{ENV['BUFFER_QUEUE_FULL_ACTION'] || 'block'}"
        </buffer>
      </match>
    </label>
4. Check the fluentd pod status
 oc logs fluentd-2khbm
2020-07-14 17:40:52 +0000 [error]: unexpected error error_class=Errno::ENOENT error="No such file or directory @ rb_sysopen - /var/run/ocp-collector/secrets/kafka/tls.crt"
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluent-plugin-kafka-0.13.0/lib/fluent/plugin/kafka_plugin_util.rb:41:in `read'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluent-plugin-kafka-0.13.0/lib/fluent/plugin/kafka_plugin_util.rb:41:in `read_ssl_file'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluent-plugin-kafka-0.13.0/lib/fluent/plugin/out_kafka2.rb:108:in `refresh_client'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluent-plugin-kafka-0.13.0/lib/fluent/plugin/out_kafka2.rb:181:in `start'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:200:in `block in start'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:179:in `block (2 levels) in lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/agent.rb:121:in `block (2 levels) in lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/agent.rb:120:in `each'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/agent.rb:120:in `block in lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/agent.rb:113:in `each'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/agent.rb:113:in `lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:178:in `block in lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:175:in `each'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:175:in `lifecycle'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/root_agent.rb:199:in `start'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/engine.rb:248:in `start'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/engine.rb:147:in `run'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/supervisor.rb:592:in `block in run_worker'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/supervisor.rb:823:in `main_process'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/supervisor.rb:586:in `run_worker'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/lib/fluent/command/fluentd.rb:338:in `<top (required)>'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:59:in `require'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:59:in `require'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/share/gems/gems/fluentd-1.9.2/bin/fluentd:8:in `<top (required)>'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/bin/fluentd:23:in `load'
  2020-07-14 17:40:52 +0000 [error]: /opt/rh/rh-ruby25/root/usr/local/bin/fluentd:23:in `<main>'
2020-07-14 17:40:52 +0000 [error]: unexpected error error_class=Errno::ENOENT error="No such file or directory @ rb_sysopen - /var/run/ocp-collector/secrets/kafka/tls.crt"


Expected Result:
The CLO only add ssl_ca_cert when there is only ca-bundle.crt in kafka secret