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: I4ba7932d679764576a46ea599499991a5c57fc56
Closes-Bug: 1349316