MB-100: minor change in NewUpgradeBaseTest._install

Change-Id: I709cdf407f3a275e0123f780df9a730e92f7ba01
Reviewed-on: http://review.couchbase.org/24664
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>