allow setting mysql_mode though mysql::config

For phabricator I have a want to set mysql to
strict mode.  I talked to Springle who concurred
that would be good.  This makes it possible.

Example:

        class { 'mysql::config':
            sql_mode      => 'STRICT_ALL_TABLES',
        }

Change-Id: If76237cdd30e1ccb566e6e50203326448a05106c