Introduce upstream virtualbox module

For some reason we recommend people use Virtualbox 4.2.6 - in order to preserve that behaviour I have:

- Renamed the in-tree module to gds_virtualbox
- Renamed the `latest` class to 42_latest (and actually made it the latest in the 4.2 series)
- Converted everyone's manifests over
- Introduced the upstream virtualbox (4.3.6) module
- Converted myself over

Everyone should think about whether they want to upgrade to a modern virtualbox.

/cc @alphagov/repo-gds-boxen