Added option to change mysql configuration path

I wanted to add vagrant-puppet-mysql as a submodule to my repo, 
so when I added my modifications to mysql config I couldn't push it to my repo
as it was stored under submodule repo.

So I added option to set custom path for configuration.

Here is the example of usage:

```
class { 'mysql':
    root_password => "''",
    config_path => '/vagrant/configs/mysql.config',
    require => Exec['apt-get update'],
}
```