fix: enable nat gateway when only private subnets are enabled

[wadhah101]

what

• Enabled Nat gateway creation when the module does not create public IPs
• Linked the nat gateway with the private subnet instead

why

• The default resource of Nat gateway will fail if public_subnets_enabled=false due to it using the first public subnet by default

references

[joe-niland]

Hi @wadhah101 just confirming - what use case are you solving here?

[wadhah101]

Hi @joe-niland
this for the case when

module "subnetprv" {
  source  = "cloudposse/dynamic-subnets/aws"
  version = "2.4.2"

  vpc_id          = module.vpc.vpc_id
  igw_id          = [module.vpc.igw_id]
  ipv4_cidr_block = ["10.10.1.0/24"]

  public_route_table_enabled             = false
  public_open_network_acl_enabled        = false
  ipv4_public_instance_hostnames_enabled = false

  nat_gateway_enabled = true
  public_subnets_enabled  = false
  private_subnets_enabled = true  # the issue is when only private subnets are enabled

  metadata_http_endpoint_enabled = true
  availability_zones             = data.aws_availability_zones.available.names

  context = module.label.context
}

The module will crash when trying to create the nat_gateway, because it's hardcoded to use the public subnets only.
But the gateway default config says it should be enabled even for when we only have private subnets.