keep old update repo versions
as last line of defense against broken online updates (https://bugzilla.opensuse.org/show_bug.cgi?id=968877) we could make pontifex keep a certain amount of old online update repo versions. That way we could revert to previous metadata more easily.
- find out how syning of the online update repo to pontifex works
- figure out how to store previous version in practice without causing extra traffic