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>