MB-10164: log reasons for the shard count choice

Change-Id: Iea57eea88167e1f00949a35051d632c09ec8db5e
Reviewed-on: http://review.couchbase.org/34869
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>