Add APT Support

[VoltaireNoir]

This PR adds Advanced Package Tool (APT) support to genpack

Things to note:

• The implementation uses apt-get for the most part but relies on apt for search and list_installed
• add_repo implementation directly writes to the sources.list file without any repo string validation
  • Whether genpack should support some kind of validation for repo's should be considered in the future
• Integration tests require elevated privileges to work. This is tricky in the CI environment, and therefore the tests are currently ignored.