Silence start_mock.bat command output

Change-Id: I52536d7029c23e2ab8ec4dad202da25574fb046a
Reviewed-on: http://review.couchbase.org/28005
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>