Apply my recent changes to nightly-modular.sh on 27 branch
    
    This applies the same changes I recently submitted for the
    nightly compose scripts to nightly-modular.sh on the 27 branch,
    which is used for the Fedora-Modular 27 nightly composes. So
    it makes that script include the shortname in fedmsgs and
    simplifies the release variables etc. just as for the other
    scripts.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>