MB-100: revised rebalance in test

Change-Id: I37ab9df09e90a2a6ac94d65220b7652b1326f8a6
Reviewed-on: http://review.couchbase.org/27648
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>