CBQE-1003L fix in UpgradeTests.tearDown

Change-Id: Ifd3b56e071e0c3adaef21172348d7978816c9012
Reviewed-on: http://review.couchbase.org/25035
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>