Description of problem:
Documentation indicates that by default dynamic bucket resharding is enabled by default in Luminous release. I installed RHCS 3.1 GA software, ran RGW workloads
and was not able to invoke this activity.

Version-Release number of selected component (if applicable):
RHCS 3.1 ceph version 12.2.5-42.el7cp (82d52d7efa6edec70f6a0fc306f40b89265535fb) luminous (stable)

How reproducible:
Always

Steps to Reproduce:
1. The documentation here
http://docs.ceph.com/docs/mimic/radosgw/dynamicresharding/ states
  Enable/Disable Dynamic bucket index resharding:
  -rgw_dynamic_resharding: true/false, default: true.
  rgw_max_objs_per_shard: maximum number of objects per bucket index shard, 
       default: 100000 objects.
2. Monitored workload which created a pool with five buckets and 22M objects total using this cmd I noticed the queue is always empty
  # radosgw-admin reshard list
3. Ran other workloads which created more objects and never saw any entries
in 'reshard list' output

Actual results:
Running various workloads I never saw the 'reshard list' cmd have any entries

Expected results:
  As stated in the documentation:
<BEGIN>
Each bucket index shard can handle its entries efficiently up until reaching a certain threshold of entries. If this threshold is exceeded the system could encounter performance issues. The dynamic resharding feature detects this situation and increases automatically the number of shards used by the bucket index, resulting in the reduction of the number of entries in each bucket index shard. This process is transparent to the user.

The detection process runs: 1. When new objects are added to the bucket 2. In a background process that periodically scans all the buckets This is needed in order to deal with existng buckets in the system that are not being updated.
<END> 

Additional info:
On my cluster dynamic bucket sharding related settings have these values:

    "rgw_dynamic_resharding": "true"
    "rgw_override_bucket_index_max_shards": "0"

which leaves the feature disabled as indicated by "num_shards":-1 :
radosgw-admin reshard status --bucket mycontainers5
[
    {
        "reshard_status": 0,
        "new_bucket_instance_id": "",
        "num_shards": -1
    },
    ...  <repeated four times> ....
]