CBQE-0: fix for negative tests for rebalance

Change-Id: Iac2e6c39b01b472e73dd95f0a166669a70c971ae
Reviewed-on: http://review.couchbase.org/28113
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>