openqa/server: reduce PPC update group asset size
    
    We need to treat it and the x86_64 update group separately to
    do this, but it really doesn't need 200G. We have images from
    three weeks ago, and we don't need that kind of buffer, and space
    is a bit tight.
    
    Note: there is no aarch64 updates group as we do not currently
    run updates tests on aarch64.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>