CBQE-2026: add simple UI tests for graceful failover

Change-Id: Ie9c9f0e75889ffe4e4b3231c724f1cf494a9bc42
Reviewed-on: http://review.couchbase.org/34987
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>