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}"]
 }