✨ #42 Add support for provisioning only a single NAT Gateway

[n8io]

DEPENDENT ON PR terraform-community-modules#57

Sample setup:

• You have 2 public subnets
• You have 2 private subnets

Scenario 1 (high availability)

Prior to this change when you set enable_nat_gateway=true you would get the following infrastructure:

• 2 EIPs (1 per private subnet)
• 2 NAT Gateway (1 per private subnet)

While this follows high availability best practices and should be your configuration in a production, sometimes you don't want to incur the cost of having multiple NAT gateways in your dev and testing environments.

Scenario 2 (low availability)

After this change when you set enable_nat_gateway=true and single_nat_gateway=true you would get the following infrastructure:

• 1 EIP (1 total)
• 1 NAT Gateway (1 total)

Meaning that you share a single EIP/NAT gateway pairing across all of your private subnets. Reducing costs while maintaining functionality but being far less available. This leaves the onus on the person terraforming to decide whether or not they are ok with the cost/risk tradeoff.

Again, this scenario is not intended for a production environment.