IPv4 NAT function

```
What steps will reproduce the problem?
1. In the GUI, click on a node then click on "services", then look
under the "Security" menu
2. In the Security menu, it would be nice to have a radio button
for "IPv4 NAT" (Network Address translator)
3.

What is the expected output? What do you see instead?

Would like to have a separate radio button for IPv4 NAT, even
though the configuration lines could easily be added under one
of the other radio buttons such as "Firewall".

What version of the product are you using? On what operating system?

Latest SVN bits as of 05/31/2013, on ubuntu 12.04

Please provide any additional information below.

Example configuration lines:

# IPv4 NAT
/sbin/iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
/sbin/iptables -A FORWARD -i eth1 -o eth0 -m state --state RELATED,ESTABLISHED
-j ACCEPT
/sbin/iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT

```

Original issue reported on code.google.com by `fltemp...@gmail.com` on 31 May 2013 at 5:58