Add OpenBSD support.

This is a first step in OpenBSD support, tested with mysql-server-5.1.73p0v0 package on OpenBSD 5.6-current, using the following configuration stored in hiera.

mysql::server::root_password: topsecret
mysql::server::manage_config_file: true
mysql::server::purge_conf_dir: true
mysql::server::package_ensure: present
mysql::server::service_enabled: true
mysql::server::remove_default_accounts: true
mysql::server::users:
  'dashboard@localhost':
    ensure: present
    max_connections_per_hour: 0
    max_queries_per_hour: 0
    max_updates_per_hour: 0
    max_user_connections: 0
    password_hash: '*TOPSECRETTOPSECRETTOPSECRET'
mysql::server::grants:
  'dashboard@localhost/dashboard_production.*':
    ensure: present
    options:
      - GRANT
    privileges:
      - ALL
    table: 'dashboard_production.*'
    user: dashboard@localhost
mysql::server::databases:
  dashboard_production:
    ensure: present
    charset: utf8
mysql::server::override_options:
  mysqld:
    user: _mysql
    datadir: /var/mysql


The odds are that I have to use the mysql::server::override_options :(
Fixing that, is for another diff, in order to keep this one clean and easier to review.

The change to server/service.pp is because there is no 'mysql' user and group, but _mysql user and _mysql group.
Another solution would be to add a $mysqlgroup variable to mysql::params list and use that. 
However, the $mysql::params::log_error file is owned by _mysql:_mysql on my OpenBSD system instead of the root_group.

This is only a first step, that allowed me to create a database. 
 - Further steps I'd look into would be:
   * get rid of the currently required usage of override_options
   * introduce a $mysqluser and $mysqlgroup to params.pp and use those throughout the classes
   * in other places, i.e. the backup config, there the group root is hardcoded, but on OpenBSD there is no group root, but wheel, so use the $root_group there