Remove asset_dir variable and optional asset writes

* Originally, poseidon/terraform-render-bootstrap generated
TLS certificates, manifests, and cluster "assets" written
to local disk (`asset_dir`) during terraform apply cluster
bootstrap
* Typhoon v1.17.0 introduced bootstrapping using only Terraform
state to store cluster assets, to avoid ever writing sensitive
materials to disk and improve automated use-cases. `asset_dir`
was changed to optional and defaulted to "" (no writes)
* Typhoon v1.18.0 deprecated the `asset_dir` variable, removed
docs, and announced it would be deleted in future.
* Add Terraform output `assets_dir` map
* Remove the `asset_dir` variable

Cluster assets are now stored in Terraform state only. For those
who wish to write those assets to local files, this is possible
doing so explicitly.

```
resource local_file "assets" {
  for_each = module.yavin.assets_dist
  filename = "some-assets/${each.key}"
  content = each.value
}
```

Related:

* poseidon/typhoon#595
* poseidon/typhoon#678

bootstrap.tf

@@ -1,11 +1,10 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=7988fb7159cb81e2d080b365b147fe90542fd258"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=9037d7311b949439b217cd9c657d4500eab3e16b"
 
   cluster_name                    = var.cluster_name
   api_servers                     = [var.k8s_domain_name]
   etcd_servers                    = var.controllers.*.domain
-  asset_dir                       = var.asset_dir
   networking                      = var.networking
   network_mtu                     = var.network_mtu
   network_ip_autodetection_method = var.network_ip_autodetection_method

outputs.tf

@@ -2,3 +2,9 @@ output "kubeconfig-admin" {
   value = module.bootstrap.kubeconfig-admin
 }
 
+# Outputs for debug
+
+output "assets_dist" {
+  value = module.bootstrap.assets_dist
+}
+

variables.tf

@@ -152,12 +152,6 @@ variable "enable_aggregation" {
 
 # unofficial, undocumented, unsupported
 
-variable "asset_dir" {
-  type        = string
-  description = "Absolute path to a directory where generated assets should be placed (contains secrets)"
-  default     = ""
-}
-
 variable "cluster_domain_suffix" {
   type        = string
   description = "Queries for domains with the suffix will be answered by coredns. Default is cluster.local (e.g. foo.default.svc.cluster.local) "