CBQE-928: minor fix with upgrade with warmup

Change-Id: I475aece46ac562f5766f05c4fd778d39e46dc3f6
Reviewed-on: http://review.couchbase.org/23709
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>