atomic: use the dl.fp.o url
    
    Would be better to just go ahead and use the dl.fp.o url in our
    ostreesetup command so that we don't have to worry about doing the
    remote delete/add in the %post. The dl.fp.o url [1] is just a redirect
    to the kojipkgs url [2] anyway.
    
    [1] https://dl.fedoraproject.org/pub/fedora/linux/atomic/25/
    [2] https://kojipkgs.fedoraproject.org/atomic/25/