CBQE-927: offline_cluster_upgrade with warmup

Change-Id: Ie2fd530e6dc87126f8720697764cd7a0b59ab15a
Reviewed-on: http://review.couchbase.org/23685
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>