CBQE:0 Support graceful failover in xdcr tests

Change-Id: I75779d1b94a9ea0ec327fde70dc21a300b454a53
Reviewed-on: http://review.couchbase.org/41328
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>