add teardown method to clean up cluster
after running tests

Change-Id: Id0c24279f2b4a0a8350a1b3e2f56cb339cd9c96f
Reviewed-on: http://review.couchbase.org/34584
Tested-by: Thuan Nguyen <soccon@gmail.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>