Enable build network masquerading on symphony-gateway

[sierra-tango-echo]

Enable build network masquerading on symphony-gateway so that we can use the gateway for NAT magic

[sierra-tango-echo]

#BUILD NETWORK FORWARDING TO EXTERNAL NETWORK
-A FORWARD -i ext -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth0 -o ext -j ACCEPT