Code review comment for lp:~paelzer/curtin/bug-1574113-derived-repositories

This now also includes some extensions/fixes created and discussed on the cloud-init side of things.
There is no reason to wait and find them again in the curtin code.

I could build fine in sbuild, but was unable to test in a ppa for now (gets rejected for bin/source mismatch for some reason).

I started to mess around with the changes file trying to fix it, but I guess smoser has a much nicer and faster way to throw that at a ppa - I'd be happy about a discussion what was missing.

Ready for another round of review for getting merged.