CBQE-2219: fix incremental_rebalance_in_with_ops

run load data and rebalance async, firstly wait rebalance /failed to avoid test hanging
http://qa.hq.northscale.net/view/3.0.0/job/centos_x64--02_04--Rebalance_tests_UPR-P0/37/consoleText

Change-Id: I72b072ee05343d9b6753cbc2115d8e817410f7a5
Reviewed-on: http://review.couchbase.org/36658
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>