Enable koji-flatpak plugin for staging
Just for staging for now:
- enable 'podman-login' role for the buildvm group, so built Flatpaks
can be pushed to the skopeo registry.
- add the koji-flatpak hub plugin to koji_hub role
- add the koji-flatpak builder plugin to the koji_builder role
and configure it.
Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>