apt_repository: do not fetch Ubuntu PPA signing keys if PPA is already added

This solution greatly speeds up consequent uses of apt_repository for PPAs by skipping two extra network calls (fetching PPA info and fetching the actual signing keys).
The assumption is that we don't need to fetch the keys if the corresponding sources file already exists.

While this approach works for me, it feels kind of hackish.
The right solution would be to capture and propagate "changed" state from `_add_valid_source` and act accordingly, but I'm not brave enough to refactor this :)