Don't schedule PowerPC jobs by default, for now
    
    We have no PowerPC workers on prod, but now we merged the PPC
    changes, PPC jobs are getting scheduled and piling up. We don't
    want that. For now, let's remove ppc64 images from the stock
    WANTED list, and to keep PowerPC tests running on staging, I'll
    give it an override file for the WANTED list via ansible.