atomic/host: Update to use new ISO model (in fedora.conf)
    
    This is a rework of <https://pagure.io/pungi-fedora/pull-request/108>,
    and re-applied from <https://pagure.io/pungi-fedora/pull-request/167>
    since I found out that in fact `fedora-atomic.conf` isn't used in
    this branch.
    Note that I left a few things like git branches pointed at f26, but
    I think these should be master most likely.
    
    Signed-off-by: Colin Walters <walters@verbum.org>