metadata archive: include labels in series file
    
    I went back to fedfind to try and rework it around the new archive
    stuff, and realized we're still missing a way to do cid_from_label
    - that is, if we know a compose's label and release, work out its
    compose ID.
    
    Here's a way to do that! For labelled composes, we write the
    label as well as the compose ID to the series file. So if you need
    to do cid_from_label you can just read the series file, find the
    line with the label you want on it, and read the first word in
    that line.
    
    If you just want the series of compose IDs, you just have to
    split each line and take only the first word.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>