Add tests for multi bucket - swap rebalances

Change-Id: Iddb025ed828bc5f39f15e498a8f9dba2d43c5c99
Reviewed-on: http://review.couchbase.org/30427
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Ketaki Gangal <ketakigangal@gmail.com>