Allow overriding NAT Gateway for fallback (#89)

It may be the case that only NAT Gateway is kept
for fallback from alternat instances, since paying
for separate gateways per AZ may not be worth the
extra `NatGatewayHours` costs. In such case, it can
be useful to manage the single NAT Gateway outside
of the module, and provide its ID to Lambda functions:
```
resource "aws_nat_gateway" "this" {}

module "alternat" {
  lambda_environment_variables = {
    NAT_GATEWAY_ID = aws_nat_gateway.this.id
  }
}
```

README.md

@@ -237,6 +237,13 @@ While we'd like for this to be available on the Terraform Registry, it requires
   ```
 - If you see errors like: `error connecting to https://www.google.com/: <urlopen error [Errno 97] Address family not supported by protocol>` in the connectivity tester logs, you can set `lambda_has_ipv6 = false`. This will cause the lambda to request IPv4 addresses only in DNS lookups.
 
+- If you want to use just a single NAT Gateway for fallback, you can create it externally and provide its ID through the `nat_gateway_id` variable. Note that you will incur cross AZ traffic charges of $0.01/GB.
+
+  ```tf
+    create_nat_gateways = false
+    nat_gateway_id      = "nat-..."
+  ```
+
 
 
 ## Future work

functions/replace-route/app.py

@@ -82,6 +82,11 @@ def get_vpc_id(route_table):
 
 
 def get_nat_gateway_id(vpc_id, subnet_id):
+    nat_gateway_id = os.getenv("NAT_GATEWAY_ID")
+    if nat_gateway_id:
+        logger.info("Using NAT_GATEWAY_ID env. variable (%s)", nat_gateway_id)
+        return nat_gateway_id
+
     try:
         nat_gateways = ec2_client.describe_nat_gateways(
             Filters=[

modules/terraform-aws-alternat/lambda.tf

@@ -36,6 +36,7 @@ resource "aws_lambda_function" "alternat_autoscaling_hook" {
   environment {
     variables = merge(
       local.autoscaling_func_env_vars,
+      { NAT_GATEWAY_ID = var.nat_gateway_id },
       var.lambda_environment_variables,
     )
   }
@@ -156,6 +157,7 @@ resource "aws_lambda_function" "alternat_connectivity_tester" {
         ROUTE_TABLE_IDS_CSV = join(",", each.value.route_table_ids),
         PUBLIC_SUBNET_ID    = each.value.public_subnet_id
         CHECK_URLS          = join(",", var.connectivity_test_check_urls)
+        NAT_GATEWAY_ID      = var.nat_gateway_id,
       },
       local.has_ipv6_env_var,
       var.lambda_environment_variables,

modules/terraform-aws-alternat/variables.tf

@@ -195,6 +195,12 @@ variable "vpc_az_maps" {
   }))
 }
 
+variable "nat_gateway_id" {
+  description = "NAT Gateway ID to use for fallback. If not provided, the gateway in the same subnet as relevant NAT instance is selected."
+  type        = string
+  default     = ""
+}
+
 variable "vpc_id" {
   description = "The ID of the VPC."
   type        = string