CBQE-1105: add rebalance progress api tests

Change-Id: I1d481c15a1d5adc0dd3cfea784717811be296c2b
Reviewed-on: http://review.couchbase.org/26111
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>