spgw should setup NAT automatically

[spencersevilla]

The SPGW is supposed to be configured to issue the correct iptables commands for NAT automatically whenever it comes up. Unfortunately, this functionality is broken (big surprise). For purposes of shipping, I wrote our own NAT script (installed to /usr/bin/spgw_nat.sh) that sets up NAT and is called by systemctl whenever the spgw is launched, but there's no reason that these two things should be logically separate. It makes more sense to just look at how the SPGW was trying to setup NAT, and then fix that work, using spgw_nat.sh as a model. spgw_config_process() is a good place to start here.