MB-100: set 20 min max for verification after upgrade

Change-Id: I3deed357759c867d18a01704cd43f4ec015b5ff1
Reviewed-on: http://review.couchbase.org/24828
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>