CBQE-927: skip upgrade teardown if test failed

Change-Id: I529edc7561b160b7758266b4b5d2642f6e558147
Reviewed-on: http://review.couchbase.org/23954
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>