[checks] Don't always require jigdo
    
    If the configuration specifically requests no jigdos, there is no point
    in checking for the binary existence.
    
    This is not 100% reliable. The jigdo option defaults to True, so if the
    option is not specified the binary is required even if there are no
    images configured.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>