run-pungi: atomic: Change the `ostree_osname` to be fedora-atomic
    
    We should be consistent with the cloud image kickstart,
    which uses `fedora-atomic`.
    
    Note this string actually appears in the user interface and may be
    typed by the admins, so the shift key gymnastics required by the
    previous value are more painful than just having it in a URL or the
    like.