Per https://tickets.puppetlabs.com/browse/MODULES-3441, the mysql
module has behaviour which varies by server version. The version is
discovered by running mysqld -V. On hosts without a MySQL server
package install, this fails, which means that contrary to the README,
it's not actually possible to use this module to manage a remote db.
This PR moves the version string discovery into a new fact,
mysqld_version which is used by the provider. This makes it possible
to configure the db version with a custom fact when a remote db
(eg AWS RDS) is being managed.
