Require Class['mysql::server'], not Service['mysqld'] when creating DB.

When using version >= 2.2 of the MySQL module, setting up
the database for Keystone depended on the MySQL service
(Service['mysqld']). However, if the service is not managed
by Puppet (e.g. it is managed through Pacemaker instead),
this would cause catalog compilation to fail because there
is no Service['mysqld']. This is the case when the MySQL
module is called with service_manage => false. Requiring
Class['mysql::server'] instead fixes this.

Change-Id: Ia1b34051dd6e4fc0916ac788d6027c1f987418a8
Closes-Bug: 1349316