Put /var/lib/mock on separate disk for live image build test
    
    The update live image build test keeps running out of disk space.
    We've bumped the minimal disk image from 12GB all the way up to
    20GB so far but it keeps happening. So let's try a different
    strategy: use a scratch disk to mount /var/lib/mock. That's where
    all the space gets used. This should allow us to reduce the size
    of the minimal disk image again, and giving it 25GB of empty disk
    should avoid it running out of space again for a while.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>