kojid: Download only 'repomd.xml'
    
    We have pretty slow connection from s390x koji which helped to uncover
    this part. Kojid downloads all files from repomd.xml (incl. filelists)
    which is really big. What we really want is just find location of
    'origin' (used later in the code).
    
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>