Underestimate queued_chunks_limit_size value with chunkLimitSize and totalLimitSize tuning parameters

Description of problem:

When configuring chunkLimitSize and totalLimitSize for tuning the performance of the Fluentd log forwarder, queued_chunks_limit_size is underestimated unintentionally.
Because the tuned totalLimitSize value is ignored and the default totalLimitSize(`15% of Disk` / `num of Outputs`) is picked for calculating queued_chunks_limit_size.

$ oc get ClusterLogging instance -o yaml
spec:
  forwarder:
    fluentd:
      buffer:
        chunkLimitSize: 8m
        totalLimitSize: 2G  
$ oc logs fluentd-xxx
Setting each total_size_limit for 1 buffers to 4910635622 bytes
Setting queued_chunks_limit_size for each buffer to 585
Setting chunk_limit_size for each buffer to 8388608

$ oc get ClusterLogging instance -o yaml
spec:
  forwarder:
    fluentd:
      buffer:
        chunkLimitSize: 1G
        totalLimitSize: 256G
$ oc logs fluentd-yyy
Setting each total_size_limit for 1 buffers to 4910635622 bytes
Setting queued_chunks_limit_size for each buffer to 4
Setting chunk_limit_size for each buffer to 1073741824

This underestimated queued_chunks_limit_size value will cause performance degradation issues.

Besides, total_size_limit in the fluend pod log doesn't match the configured total_size_limitin the generated flunet.conf.

The following part of fluent.conf is quoted from fluend-yyy.
fluentd-yyy says total_size_limit is 4910635622, but the configured value is 256G. 

<label @FLUENTD_INFRA>
  <match **>
    # https://docs.fluentd.org/v1.0/articles/in_forward
    @type forward
    heartbeat_type none
    keepalive true

    <buffer>
      @type file
      path '/var/lib/fluentd/fluentd_infra'
      queued_chunks_limit_size "#{ENV['BUFFER_QUEUE_LIMIT'] || '1024' }"
      total_limit_size 256G
      chunk_limit_size 1G

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

The latest code from https://github.com/openshift/cluster-logging-operator master:

  $ git log --oneline
  2c48ecbc Merge pull request #1021 from vimalk78/log-1355

How reproducible:

Always

Steps to Reproduce:

1. Deploy CLO
2. Create CLF/instance
3. Edit the ClusterLogging CR

     $ oc edit ClusterLogging instance
     spec:
       forwarder:
         fluentd:
           buffer:
             chunkLimitSize: 1G
             totalLimitSize: 256G
   

1. Redeploy fluentd pods
2. Check queued_chunks_limit_size in the fluentd config map and "oc logs fluentd

Actual results

Please check it in "Description of problem".

Expected results:

There are two options:

1. Calculate the max number of queued chunks based on the configured chunk_limit_size and total_limit_size
2. Allow users to tune queued_chunks_limit_size as well as chunk_limit_size and total_limit_size

Additional info: