openqa/server: try setting a limit on test result disk usage
    
    We're having issues with test results eating up all the disk
    space we can throw at them (prod is over 4T, stg is over 2T -
    I don't know why prod is bigger, that's odd, but it may be an
    odd effect of having more arches on stg, maybe aarch64 and
    ppc64le tests generally have smaller videos, or something).
    This config setting should make openQA keep the space usage
    on the partition at a max of 85%, by deleting videos from older
    tests as required.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>