no longer require availability_zones argument

main.tf

-variable "availability_zones" {
-  description = "List of availability zone in which the ECS cluster should reside"
-  type        = "list"
-}
-
 variable "vpc_id" {
   description = "ID of the VPC in which the ECS cluster should reside"
 }
 
 variable "subnet_ids" {
   description = "List of subnets in which the ECS cluster should reside"
-  type        = "list"
+  type        = list
 }
 
 variable "image_id" {
@@ -34,7 +29,7 @@ variable "bastion_instances" {
 
 resource "aws_security_group" "bastion" {
   name        = "Bastion"
-  vpc_id      = "${var.vpc_id}"
+  vpc_id      = var.vpc_id
   description = "Allows traffic from and to the Bastion EC2 instance"
 
   ingress {
@@ -68,39 +63,40 @@ resource "aws_security_group" "bastion" {
 
 data "template_file" "userdata" {
   template = "${file("${path.module}/userdata.yml")}"
-  vars {}
+  vars     = {}
 }
 
 resource "aws_launch_configuration" "bastion" {
-  name_prefix = "${format("%s-", "Bastion")}"
+  name_prefix = format("%s-", "Bastion")
 
-  image_id        = "${var.image_id}"
-  instance_type   = "${var.instance_type}"
-  key_name        = "${var.key_name}"
-  security_groups = ["${aws_security_group.bastion.id}"]
-  user_data       = "${data.template_file.userdata.rendered}"
+  image_id        = var.image_id
+  instance_type   = var.instance_type
+  key_name        = var.key_name
+  security_groups = [aws_security_group.bastion.id]
+  user_data       = data.template_file.userdata.rendered
 
   lifecycle {
     create_before_destroy = true
     ignore_changes = [
-      "image_id",
+      image_id,
     ]
   }
 }
 
 resource "aws_autoscaling_group" "bastion" {
   name                 = "Bastion - ${aws_launch_configuration.bastion.name}"
-  vpc_zone_identifier  = ["${var.subnet_ids}"]
-  launch_configuration = "${aws_launch_configuration.bastion.id}"
-  min_size             = "${var.bastion_instances}"
-  max_size             = "${var.bastion_instances}"
-  desired_capacity     = "${var.bastion_instances}"
+  vpc_zone_identifier  = var.subnet_ids
+  launch_configuration = aws_launch_configuration.bastion.id
+  min_size             = var.bastion_instances
+  max_size             = var.bastion_instances
+  desired_capacity     = var.bastion_instances
 
   tag {
     key                 = "Name"
     value               = "Bastion"
     propagate_at_launch = true
   }
+
   tag {
     key                 = "service"
     value               = "bastion"