MB-100: needless Cluster() in basetc for upgrade tests

Change-Id: Ia4649314a8a0a2ca5d74b47a2050159fc629d937
Reviewed-on: http://review.couchbase.org/23908
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>