add fargate_subnets variable

The EKS cluster can be provisioned with both private and public subnets.
But Fargate only accepts private ones.

This new variable allows to override the subnets to explicitly pass the
private subnets to Fargate and work around that issue.

Author: zimbatm

fargate.tf

@@ -7,7 +7,7 @@ module "fargate" {
   fargate_profiles                  = var.fargate_profiles
   iam_path                          = var.iam_path
   iam_policy_arn_prefix             = local.policy_arn_prefix
-  subnets                           = var.subnets
+  subnets                           = length(var.fargate_subnets) > 0 ? var.fargate_subnets : var.subnets
   tags                              = var.tags
 
   # Hack to ensure ordering of resource creation.

variables.tf

@@ -356,6 +356,12 @@ variable "fargate_profiles" {
   default     = {}
 }
 
+variable "fargate_subnets" {
+  description = "A list of private subnets to use for Fargate. Defaults to the subnets var if empty."
+  type        = list(string)
+  default     = []
+}
+
 variable "create_fargate_pod_execution_role" {
   description = "Controls if the EKS Fargate pod execution IAM role should be created."
   type        = bool