Make MySQL storage persistent

[eelkeblok]

This pull request adds persistent storage for MySQL using the vagrant persistent storage plugin. (The changes to the Vagrantfile are thus that the plugin is not required, things will continue to function like they did before when the plugin is not installed).

[r8]

Thank you, very nice idea.
I will definitely use it little later, but I will not merge it for now. I think I would prefer to manage it per-instance basis, not enabling globally by installing the plugin.