Add/update docs for asset_dir and kubeconfig usage

* Original tutorials favored including the platform (e.g.
google-cloud) in modules (e.g. google-cloud-yavin). Prefer
naming conventions where each module / cluster has a simple
name (e.g. yavin) since the platform is usually redundant
* Retain the example cluster naming themes per platform

bare-metal/fedora-coreos/kubernetes/variables.tf

@@ -71,6 +71,7 @@ variable "ssh_authorized_key" {
 variable "asset_dir" {
   type        = string
   description = "Absolute path to a directory where generated assets should be placed (contains secrets)"
+  default     = ""
 }
 
 variable "networking" {