common/options: Change HDD OSD shard configuration defaults for mClock

common/options: Change HDD OSD shard configuration defaults for mClock

Based on tests performed at scale on a HDD based cluster, it was found
that scheduling with mClock was not optimal with multiple OSD shards. For
e.g., in the scaled cluster with multiple OSD node failures, the client
throughput was found to be inconsistent across test runs coupled with
multiple reported slow requests.

However, the same test with a single OSD shard and with multiple worker
threads yielded significantly better results in terms of consistency of
client and recovery throughput across multiple test runs.

For more details see https://tracker.ceph.com/issues/66289.

Therefore, as an interim measure until the issue with multiple OSD shards
(or multiple mClock queues per OSD) is investigated and fixed, the
following change to the default HDD OSD shard configuration is made:

 - osd_op_num_shards_hdd = 1 (was 5)
 - osd_op_num_threads_per_shard_hdd = 5 (was 1)

The other changes in this commit include:
 - Doc change to the OSD and mClock config reference describing
   this change.
 - OSD troubleshooting entry on the procedure to change the shard
   configuration for clusters affected by this issue running on older
   releases.
 - Add release note for this change.

Fixes: https://tracker.ceph.com/issues/66289
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
(cherry picked from commit 0d81e721378e6d7a647c5a4f6aab3cede1a828d3)

Conflicts:
        doc/rados/troubleshooting/troubleshooting-osd.rst
- Included the troubleshooting entry before the "Flapping OSDs" section.
        PendingReleaseNotes
- Moved the release note under 18.2.4 section and removed unrelated entries