Make the NAT optional

[onetwopunch]

Many users have strict requirements about systems reaching out to the internet. Private Google Access alleviates some of these concerns, but in this module, the nat, external IP are generated by default with no way to override.

To address these concerns, we should:

• Make the NAT, external IP and router optional using a flag variable. Something like allow_public_egress
• If there is no NAT, we need to adjust the startup script to not make external curl calls. Possibly pass in a http_proxy to the script