Rename CLC files and favor Terraform list index syntax

* Rename Container Linux Config (CLC) files to *.yaml to align
with Fedora CoreOS Config (FCC) files and for syntax highlighting
* Replace common uses of Terraform `element` (which wraps around)
with `list[index]` syntax to surface index errors

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

@@ -1,7 +1,7 @@
 locals {
   # Channel for a Container Linux derivative
   # coreos-stable -> Container Linux Stable
-  channel = element(split("-", var.os_image), 1)
+  channel = split("-", var.os_image)[1]
 }
 
 # Workers scale set
@@ -104,7 +104,7 @@ data "ct_config" "worker-ignition" {
 
 # Worker Container Linux configs
 data "template_file" "worker-config" {
-  template = file("${path.module}/cl/worker.yaml.tmpl")
+  template = file("${path.module}/cl/worker.yaml")
 
   vars = {
     kubeconfig             = indent(10, var.kubeconfig)