Add Container Linux Config snippets feature

* Introduce the ability to support Container Linux Config
"snippets" for controllers and workers on cloud platforms.
This allows end-users to customize hosts by providing Container
Linux configs that are additively merged into the base configs
defined by Typhoon. Config snippets are validated, merged, and
show any errors during `terraform plan`
* Example uses include adding systemd units, network configs,
mounts, files, raid arrays, or other disk provisioning features
provided by Container Linux Configs (using Ignition low-level)
* Requires terraform-provider-ct v0.2.1 plugin

aws/container-linux/kubernetes/controllers.tf

@@ -78,4 +78,5 @@ data "ct_config" "controller_ign" {
   count        = "${var.controller_count}"
   content      = "${element(data.template_file.controller_config.*.rendered, count.index)}"
   pretty_print = false
+  snippets     = ["${var.controller_clc_snippets}"]
 }

aws/container-linux/kubernetes/variables.tf

@@ -60,6 +60,18 @@ variable "worker_type" {
   description = "Worker EC2 instance type"
 }
 
+variable "controller_clc_snippets" {
+  type        = "list"
+  description = "Controller Container Linux Config snippets"
+  default     = []
+}
+
+variable "worker_clc_snippets" {
+  type        = "list"
+  description = "Worker Container Linux Config snippets"
+  default     = []
+}
+
 # bootkube assets
 
 variable "asset_dir" {

aws/container-linux/kubernetes/workers.tf

@@ -16,4 +16,5 @@ module "workers" {
   ssh_authorized_key    = "${var.ssh_authorized_key}"
   service_cidr          = "${var.service_cidr}"
   cluster_domain_suffix = "${var.cluster_domain_suffix}"
+  clc_snippets          = "${var.worker_clc_snippets}"
 }

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

@@ -71,3 +71,9 @@ variable "cluster_domain_suffix" {
   type        = "string"
   default     = "cluster.local"
 }
+
+variable "clc_snippets" {
+  type        = "list"
+  description = "Container Linux Config snippets"
+  default     = []
+}

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

@@ -71,4 +71,5 @@ data "template_file" "worker_config" {
 data "ct_config" "worker_ign" {
   content      = "${data.template_file.worker_config.rendered}"
   pretty_print = false
+  snippets     = ["${var.clc_snippets}"]
 }