CBQE-928: offline_cluster_upgrade with failover

Change-Id: Ic90a5ea3f240af74793a2c41de09e314af1620a7
Reviewed-on: http://review.couchbase.org/23773
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>