MB-7518: Only snapshot vbuckets from a particular shard

Change-Id: Ic31f97d6f6dfae76af25e3a70fbb0078db03f1d2
Reviewed-on: http://review.couchbase.org/26031
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>