Description of problem:

When the `chunkLimitSize` for fluentd is configured according to the documentation. The wrong chunk_limit_size is shown in the logs

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

How reproducible:


Steps to Reproduce:
1. Deploy cluster-logging
2. Set chunk_size_limit for fluentd
~~~~
# oc edit ClusterLogging instance
 spec:
  forwarder:
    fluentd:
      buffer:
        chunkLimitSize: 750k
~~~~
3. Verify that it is applied
~~~~
# oc extract configmap/fluentd --confirm
# grep chunk fluent.conf
~~~~

4. View logs
~~~~
# oc get pods   | grep fluent                                                                                                                                                                                                   ✭
NAME                                            READY   STATUS      RESTARTS   AGE

fluentd-4t5n7                                   1/1     Running     0          6m18s
fluentd-dkjhr                                   1/1     Running     0          8m11s
fluentd-fzcqb                                   1/1     Running     0          7m26s
fluentd-l5tr5                                   1/1     Running     0          6m58s
fluentd-pf2td                                   1/1     Running     0          7m50s
fluentd-tvjks                                   1/1     Running     0          6m42s
kibana-5db88c8cf-6w4t5                          2/2     Running     0          2d15h
# oc logs fluentd-4t5n7 | head -3                                                                                                                                                                                  ✭
Setting each total_size_limit for 2 buffers to 9621286195 bytes
Setting queued_chunks_limit_size for each buffer to 1146
Setting chunk_limit_size for each buffer to 8388608
~~~~

Actual results:
Setting each total_size_limit for 2 buffers to 9621286195 bytes
Setting queued_chunks_limit_size for each buffer to 1146
Setting chunk_limit_size for each buffer to 8388608

Expected results:
Setting each total_size_limit for 2 buffers to 9621286195 bytes
Setting queued_chunks_limit_size for each buffer to 1146
Setting chunk_limit_size for each buffer to 768000

Additional info:
* `run.sh` is still calculating it depending on the ${BUFFER_SIZE_LIMIT}` variable, which is no longer applied.

BUFFER_SIZE_LIMIT=$(echo ${BUFFER_SIZE_LIMIT:-8m} |  sed -e "s/[Kk]/*1024/g;s/[Mm]/*1024*1024/g;s/[Gg]/*1024*1024*1024/g;s/i//g" | bc)
BUFFER_QUEUE_LIMIT=$(expr $TOTAL_LIMIT_SIZE / $BUFFER_SIZE_LIMIT)
echo "Setting queued_chunks_limit_size for each buffer to $BUFFER_QUEUE_LIMIT"
export BUFFER_QUEUE_LIMIT
echo "Setting chunk_limit_size for each buffer to $BUFFER_SIZE_LIMIT"
export BUFFER_SIZE_LIMIT