Allow Flatcar Linux os_image on AWS, rename os_channel

* Replace os_channel variable with os_image to align naming
across clouds. Users who set this option to stable, beta, or
alpha should now set os_image to coreos-stable, coreos-beta,
or coreos-alpha.
* Default os_image to coreos-stable. This continues to use
the most recent image from the stable channel as always.
* Allow Container Linux derivative Flatcar Linux by setting
os_image to `flatcar-stable`, `flatcar-beta`, `flatcar-alpha`

aws/container-linux/kubernetes/workers/ami.tf

@@ -1,3 +1,12 @@
+locals {
+  # Pick a CoreOS Container Linux derivative
+  # coreos-stable -> Container Linux AMI
+  # flatcar-stable -> Flatcar Linux AMI
+  ami_id = "${local.flavor == "flatcar" ? data.aws_ami.flatcar.image_id : data.aws_ami.coreos.image_id}"
+  flavor = "${element(split("-", var.os_image), 0)}"
+  channel = "${element(split("-", var.os_image), 1)}"
+}
+
 data "aws_ami" "coreos" {
   most_recent = true
   owners      = ["595879546273"]
@@ -14,6 +23,27 @@ data "aws_ami" "coreos" {
 
   filter {
     name   = "name"
-    values = ["CoreOS-${var.os_channel}-*"]
+    values = ["CoreOS-${local.channel}-*"]
   }
 }
+
+data "aws_ami" "flatcar" {
+  most_recent = true
+  owners      = ["075585003325"]
+
+  filter {
+    name   = "architecture"
+    values = ["x86_64"]
+  }
+
+  filter {
+    name   = "virtualization-type"
+    values = ["hvm"]
+  }
+
+  filter {
+    name   = "name"
+    values = ["Flatcar-${local.channel}-*"]
+  }
+}
+

aws/container-linux/kubernetes/workers/variables.tf

@@ -34,10 +34,10 @@ variable "instance_type" {
   description = "EC2 instance type"
 }
 
-variable "os_channel" {
+variable "os_image" {
   type        = "string"
-  default     = "stable"
-  description = "Container Linux AMI channel (stable, beta, alpha)"
+  default     = "coreos-stable"
+  description = "AMI channel for a Container Linux derivative (coreos-stable, coreos-beta, coreos-alpha, flatcar-stable, flatcar-beta, flatcar-alpha)"
 }
 
 variable "disk_size" {

aws/container-linux/kubernetes/workers/workers.tf

@@ -41,7 +41,7 @@ resource "aws_autoscaling_group" "workers" {
 
 # Worker template
 resource "aws_launch_configuration" "worker" {
-  image_id      = "${data.aws_ami.coreos.image_id}"
+  image_id      = "${local.ami_id}"
   instance_type = "${var.instance_type}"
   spot_price    = "${var.spot_price}"