Missing NATGW routes in aws/GWLB-Demo

[mwkaufman]

@djspears In the README image it shows routes in the NATGW subnets back to Endpoint-Outbound. There doesn't seem to be any code or manual steps mentioned to set this up. Without it, the outbound internet traffic (from outside the local VPC, i.e. a different TGW attachment) will show aged-out in the firewall UI as there isn't a return route.