Add support for MariaDB > 10.0.0

MariaDB 10.0.0 allows for usernames up to 80 bytes, removing a long-standing restriction in MySQL.

We introduce a MariaDB provider for `mysql_user` and `mysql_grant` that is confined to `mysql --version >= 10.0.0` and introduces the feature `:long_usernames`.  Finally, we check for that feature in the types, before deciding that a username is too long.

Why?
----

mysql will happily accept a username that doesn't fit into its column, truncating it. This works "fine" for a system that is one-shot, but in a system that's supposed to be idempotent, we'll **actually** notice that something is amiss.

This PR addresses [MODULES-151](https://tickets.puppetlabs.com/browse/MODULES-151)