CBQE-1015: fix online_cluster_upgrade in xdcrupgrade

Change-Id: I93ffbd62eea68a82f0a62938321d7fe523542896
Reviewed-on: http://review.couchbase.org/25014
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>