Allow custom resources to be used for a CiviForm deployment

[dkatzz]

Description

Currently, we create all the resources needed for CiviForm through our deployment terraform scripts. However, Charlotte has a VPC (spanning across 2 subnet groups) and load balancer they'd like traffic to hit before getting to CiviForm. We should allow our deployment to be more modular, allowing entities to set a VPC, load balancer, or other resources to use and if those are set, we'll use them, and if not we'll create them ourselves.

Notes about Charlottes environment can be found here

Describe the solution you'd like.

Allow Charlotte to use the resources they have in their environment and have those hooked into the CiviForm deployment system. More broadly, allow civic entities to enter a VPC ID and load balancer that should be used instead of the ones we normally create with the deployment.

What priority is this?

This is high priority, since it will block Charlotte's launch

Done when:

Charlotte can successfully deploy within their AWS environment.