CBQE-0: bucketflush tests don't run teardown if setup failed

http://qa.hq.northscale.net/job/ubuntu_x64--10_01--bucket_flush-P0/16/console

Change-Id: I62a68feb15fdf5b54cdc6752ec44e05255bc8c24
Reviewed-on: http://review.couchbase.org/37272
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Meenakshi Goel <meenakshi.goel@globallogic.com>