CBQE-928: fix online upgrade tests

Change-Id: Ic592eccf62583e17581794bda66a52b27d628ab4
Reviewed-on: http://review.couchbase.org/24401
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>