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

aws/container-linux/kubernetes/ssh.tf

@@ -2,7 +2,7 @@ locals {
   # format assets for distribution
   assets_bundle = [
     # header with the unpack location
-    for key, value in module.bootstrap.assets_dist:
+    for key, value in module.bootstrap.assets_dist :
     format("##### %s\n%s", key, value)
   ]
 }