rgw: add radosgw-admin sub-command to set-min-shards for a bucket

rgw: add radosgw-admin sub-command to set-min-shards for a bucket

There is now a mechansim to set the minimum number of shards when a
bucket is created, and dynamic resharding adheres to that
setting. This adds the ability to modify that minimum shard count that
exists within the bucket layout of the bucket instance
object. Example:

    radosgw-admin bucket set-min-shards --bucket=<bucket> \
        --num-shards=<value>

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>