Make core libreoffice packages mandatory
    
    This makes core LibreOffice packages mandatory. That means the
    group they are in (libreoffice) will not be considered to be
    'installed' if they are removed, and importantly, image composes
    will fail (as opposed to 'succeeding', but with the packages left
    out) if they are missing or not installable. That's the main
    benefit here.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>