CBQE-1278: BucketFlushTests doesn't shutdown cluster if setup failed

CouchbaseServer-SanityTest-4Node-Centos64 hangs

Change-Id: I5a4eb5fd07957db17c97d7e25672b23d154cac3e
Reviewed-on: http://review.couchbase.org/26196
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Ketaki Gangal <ketakigangal@gmail.com>