manifests/repo/apt: Fix the apt::source.

This corrects two really weird things in the way we were managing the
APT repo and the key:

* Stop using a template.
* Switch to using `key_source` and point to the URL of the key.

Of note: WTFBBQ were we doing using a template for a file to begin with?
Merge pull request #188 from daenney/fix-apt-key

manifests/repo/apt: Fix the apt::source.