Enable customizing runroot task weight
    
    For different cases where runroot is used it's now possible to set
    custom weight. The usecase for this is to avoid one builder taking too
    many tasks. Especially buildinstall is quite resource intensive, so one
    builder taking multiple tasks at the same time leads to very slow
    compose time.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>