atomic: set up for 2wk updates
    
    This is inspired by 286c199 from the f25 branch. Basically we need
    to be able to pull from one ref [1] at the time we create the ISO, but
    configure it so that after an install using the ISO the user has an
    install that is configured to pull from another ref [2].
    
    This is how we do it in the branched composes (i.e. f25/f26). Even
    though rawhide doesn't follow this updates/stable ref model it makes
    sense for us to set up the templates like this for consistency. The
    actual values passed in for ostree_install_ref and ostree_update_ref
    can be the same.
    
    [1] fedora/26/x86_64/updates/atomic-host
    [2] fedora/26/x86_64/atomic-host
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>