MB-100 test only one node in cluster

Change-Id: I3ba283dcee3d1100df635e03287f6afd3df2fd5b
Reviewed-on: http://review.couchbase.org/27258
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Thuan Nguyen <soccon@gmail.com>