The current implementation for flatpak preinstall support won't work for CDNs. It needs to be adjusted post changes made by quay. Also add support for configuration through kickstart `--flatpak-registry-url` and remove the old hardcoded staging url.

These changes MUST be tested with the recently merged flatpak client's MR (https://gitlab.com/redhat/centos-stream/rpms/flatpak/-/merge_requests/43) which adds required support for CDN. The flatpak's code change makes completely obsoletes the changes introduced for handling certificates on CDN - they must be kept ONLY FOR SATELLITE SUPPORT.