Docs update: puppet does mysql install for gerrit

Change-Id: If88889554913f8f8dbd91d4455e0a91926e8bd5f
Reviewed-on: https://review.openstack.org/19313
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: shengjie min
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins