[WIP] Apt module cleanup

Soliciting feedback on this WIP apt module cleanup.

Primary changes:

* Use apt module instead of our custom-baked solution; greatly simplifies key management and repo declarations
* Remove nonfree attribute; everything gets non-free now

Things still to do:

* Make backports use mirrors.ocf
* Support for special desktop repos (Google/Mozilla apt repos)
* Support for weird architectures (e.g. Raspbian); might ignore this for now
* Update maelstrom (and any other?) modules which still do manual apt configuration