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/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}"]
 }