Add option to set instance_type on the launch template

This allows setting the instance type on the launch template rather than
the EKS Managed node group. This is useful because it is not possible to
change the instance types set on a managed node groups after creation.
The suggested way to work around this is to simply define the instance
type in the launch template.

aws/containers-roadmap#746 (comment)

modules/eks-managed-node-group/main.tf

@@ -170,6 +170,8 @@ resource "aws_launch_template" "this" {
     }
   }
 
+  instance_type = var.instance_type
+
   # # Set on node group instead
   # instance_type = var.launch_template_instance_type
   kernel_id = var.kernel_id

modules/eks-managed-node-group/variables.tf

@@ -222,6 +222,12 @@ variable "instance_market_options" {
   default     = {}
 }
 
+variable "instance_type" {
+  description = "Set of instance type associated with the Launch Template. Conflicts with `instance_types`"
+  type        = string
+  default     = null
+}
+
 variable "maintenance_options" {
   description = "The maintenance options for the instance"
   type        = any
@@ -351,7 +357,7 @@ variable "force_update_version" {
 }
 
 variable "instance_types" {
-  description = "Set of instance types associated with the EKS Node Group. Defaults to `[\"t3.medium\"]`"
+  description = "Set of instance types associated with the EKS Node Group. Defaults to `[\"t3.medium\"]`. Conflicts with `instance_types`"
   type        = list(string)
   default     = null
 }