Bug 1867129

Description of problem:
No default values are set in fluent.conf for forwarder.fluentd.buffer

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

How reproducible:
Always

Steps to Reproduce:
1. Deploy CLO and EO
2. Create clusterlogging instance with following CR:
cat <<EOF | oc create -f -
apiVersion: logging.openshift.io/v1
kind: ClusterLogging
metadata: 
  name: instance
  namespace: openshift-logging
spec: 
  collection: 
    logs: 
      fluentd:
        resources: 
          limits: 
            cpu: 250m
            memory: 1Gi
          requests: 
            cpu: 250m
            memory: 1Gi
      type: fluentd
  curation: 
    curator: 
      schedule: "*/10 * * * *"
    type: curator
  forwarder: 
    fluentd: 
      buffer: 
        flushAtShutdown: false
  logStore: 
    elasticsearch: 
      nodeCount: 1
      redundancyPolicy: ZeroRedundancy
      resources: 
        limits: 
          memory: 2Gi
        requests: 
          cpu: 100m
          memory: 1Gi
      storage: {}
    type: elasticsearch
  managementState: Managed
  visualization: 
    kibana: 
      replicas: 1
    type: kibana
EOF

3. Check the fluentd configuration:
$ oc get cm fluentd -o json | jq -r '.data."fluent.conf"'

Actual results:
    <buffer>
      @type file
      path '/var/lib/fluentd/fluentd_created_by_user'
      queued_chunks_limit_size "#{ENV['BUFFER_QUEUE_LIMIT'] || '1024' }"
      
      total_limit_size      
      chunk_limit_size "#{ENV['BUFFER_SIZE_LIMIT'] || '1m'}"
      flush_mode 
      flush_interval 
      flush_at_shutdown true
      flush_thread_count 0
      retry_type 
      retry_wait 
      retry_max_interval 
      retry_forever true
      # the systemd journald 0.0.8 input plugin will just throw away records if the buffer
      # queue limit is hit - 'block' will halt further reads and keep retrying to flush the
      # buffer to the remote - default is 'block' because in_tail handles that case
      overflow_action 
    </buffer>

Expected results:
There are default values set in the configuration