CBQE-0:: Change mutation loading logic in Rebalance tests

Change-Id: Ie88e5ea02ed4e26d1f251cabd8dc5eb0367d6735
Reviewed-on: http://review.couchbase.org/41413
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>