openqa/worker: bump load average threshold for big worker hosts
    
    This is a new feature in openQA that prevents worker hosts
    picking up new jobs if their load average is above a certain
    threshold. It defaults to 40. Our big worker hosts tend to run
    above this, so let's bump it on those.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>