CBQE-1234 Add cleanup on drainratetest

Change-Id: I24361ed4bf8b0c61e71051feb883422c5d0937e2
Reviewed-on: http://review.couchbase.org/25819
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>