MB-100: minor fix in NewUpgradeBaseTest._install

Change-Id: I53c68b4078863e253f57a67605c72250dec071fd
Reviewed-on: http://review.couchbase.org/24821
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>