MM: ignore pub/archive when searching for new versions
    
    Every time we tried to delete the s390 (without x) rawhide repository
    from MirrorManager it was re-detected as there is still a very old s390
    tree available under pub/archive. This tells MirrorManager to ignore
    everything under pub/archive when searching for new versions, releases
    and architectures.
    
    Signed-off-by: Adrian Reber <adrian@lisas.de>