CBQE-1232: test in infinite loop if _rebalance_progress return -100

RebalanceTask check status indefinitely if rest._rebalance_progress returns -100 when ServerUnavailableException was found

Change-Id: I3f3b237dc0df6f85992793719c8c327c231778ab
Reviewed-on: http://review.couchbase.org/25788
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: buildbot <build@couchbase.com>