openqa: update iptables NAT rule implementation
    
    Since we set this up, @puiterwijk added a nice `nat_rules` thing
    that lets us add NAT rules without forking the iptables template,
    and I just set up the `openqa_tap_iface` variable to avoid the
    stupid thing where I hardcoded all the possible interface names
    for different arches. So let's use those two together FOR GREAT
    JUSTICE! Or possibly just to break everything, you know, we'll
    find out shortly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>