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

Integrated the changes due to our discussion about having no "builtin" source.list.

If no custom template is provided we now fall back to modify whatever we find in the target.
All tests that should be affected passed already as well as an sbuild.

I'm running a ppa build and a full test-suite just to be sure and give this thread another ping then.