MB-100: added speed throttle to sanity test

Change-Id: I6ffdf01e6932c20af3daf1c52dab503b90261369
Reviewed-on: http://review.couchbase.org/27199
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>