CBQE-2327: Implement bucket priority in basetestcase class and add tests to rebalance and failover

Change-Id: Ib23a17ff916e68ac4b79adb73b48996f69df2424
Reviewed-on: http://review.couchbase.org/39422
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>