CBQE-0: check RebalanceHelper.rebalance_in status in swaprebalance.py

Change-Id: If6c3da628a71927b7cbce27aa7d950035fee96c1
Reviewed-on: http://review.couchbase.org/26256
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>