lowercase hostname values in qualified usernames

MySQL/MariaDB automatically downcase hostnames:

  MariaDB [mysql]> create user 'testuser'@'HOSTNAME';
  MariaDB [mysql]> select user,host from user where host = 'hostname';
  +----------+----------+
  | user     | host     |
  +----------+----------+
  | testuser | hostname |
  +----------+----------+

This causes problems when a mysql_user or datbase_user has an hostname
with non-lowercase characters:

  database_user { "root@HOSTNAME":
    ensure => absent,
  }

The SELECT statements used to determine if the user exists will fail
because the comparisons use "HOSTNAME" but the database has "hostname".
This patch forces the hostname part of "user@hostname" to lower case in
the custom type definitions.