[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 **will merge without this**
* ~~Support for special desktop repos (Google/Mozilla apt repos)~~
* Support for weird architectures (e.g. Raspbian); might ignore this for now **skipping this for now**
* ~~Update maelstrom (and any other?) modules which still do manual apt configuration~~
* Test on jessie

Tested on a few Debian stable servers and on plague (Ubuntu something).

Easiest to see [here](https://github.com/ocf/puppet/blob/apt-cleanup/modules/common/manifests/apt.pp).