Change drain rate testcase on Sanity runs

Change-Id: Ibd6dcf0256d5a345156bed64697338056eb55b08
Reviewed-on: http://review.couchbase.org/25811
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Ketaki Gangal <ketakigangal@gmail.com>