Add support for private IP address

[dmerrick]

Right now the module sets up a public IP address here.

If a user is using self-hosted github/gitlab, there is no need to expose Atlantis publicly.

It would be great if there was an option to deploy atlantis without also creating a public IP

[bschaatsbergen]

Hey @dmerrick, great suggestion! I think this could be a valuable addition to the module. For whoever implements it, note that this will likely require swapping the global load balancer for an internal one. This change could add significant bloat to the module, so we might need to consider a reference implementation or creating a separate module instead of integrating it directly into the existing one.