container: use the compose locations url for updates
    
    The secondary arches were failing because they needed:
    https://kojipkgs.fedoraproject.org/pub/fedora-secondary/updates/27/$arch/
    
    It's easier for us to use the pungi compose location urls rather than
    create separate image_build sections for the secondary architectures.
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>