CBQE-1322 resolve backup job hang problem

If some test case fails in the middle, when kicking off next
test case, we should use tearDown to clean up the buckets the
previous failed test is not able to purge.
Change-Id: I9d60ea0ceaf0d93cedbcdbaf28f4d019df2d48f3
Reviewed-on: http://review.couchbase.org/27618
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>