MB-9775: Consider groupname option for rebalance

Change-Id: Icf5a3c51635be14b5c224e61d91440eb9e1a669c
Reviewed-on: http://review.couchbase.org/31307
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>