first-run: Make --initial-setup work in minimal env, impl sample-rc.

We still don't install packages (although infra exists for it), but we
can at least install a simple sample configuration.

With the opensuse/unknown packages installed in the opensuse-leap:latest
Docker container (separately), kdesrc-build can go straight from base
image to being able to run "kdesrc-build --initial-setup" and then
immediately after run "kdesrc-build --metadata-only".

Progress!....