feat: Add Launch Template support for Managed Node Groups (#997)

NOTES: Managed Node Groups now support Launch Templates. The Launch Template it self is not managed by this module, so you have to create it by your self and pass it's id to this module. See docs and [`examples/launch_templates_with_managed_node_groups/`](https://github.com/terraform-aws-modules/terraform-aws-eks/tree/master/examples/launch_templates_with_managed_node_group) for more details.

examples/launch_templates_with_managed_node_groups/variables.tf

@@ -0,0 +1,14 @@
+variable "region" {
+  default = "eu-central-1"
+}
+
+variable "instance_type" {
+  default = "t3.small" // smallest recommended, where ~1.1Gb of 2Gb memory is available for the Kubernetes pods after ‘warming up’ Docker, Kubelet, and OS
+  type    = string
+}
+
+variable "kms_key_arn" {
+  default     = ""
+  description = "KMS key ARN to use if you want to encrypt EKS node root volumes"
+  type        = string
+}