Clean up base boxes creation scripts

- [x] Replace current Rake tasks with a simpler bash script
- [x] Introduce a `built-on` attribute on `metadata.json`
- [x] Support for debian boxes
- [x] Apply https://github.com/fgrehm/vagrant-lxc/pull/92
- [x] Move built box into `boxes/output` and clean up tmp dir after box is built
- [x] Update https://github.com/fgrehm/vagrant-lxc/wiki/Base-boxes#building-your-own
- [x] Write detailed instructions on what should be included on base box
- [x] Ensure locales are properly set up