Adds the NAT gateway to vpc-portal

Was required for prod, and dev should match prod.

Required for prod/test in order to reach the us-gov-west-1 SES endpoint.

test/aws-us-gov/mdr-test-c2/010-vpc-portal/terragrunt.hcl

@@ -38,4 +38,5 @@ inputs = {
   tags = {
     Terraform = "aws/${basename(get_parent_terragrunt_dir())}/${path_relative_to_include()}/"
   }
+  enable_nat_gateway = true # Needed for outbound LDAP access
 }