Add Cloud-Init automation with Tailscale

The commit rephrases the README to focus on Cloud-Init setup and removes
unused Ansible code, making Tailscale configuration automatic.

Subject line: Replace Ansible with automated Cloud-Init configuration

Body: Refactors infrastructure bootstrapping to leverage Cloud-Init
instead of Ansible for initial VM setup, particularly with respect to
Tailscale installation and authentication.

This simplifies the architecture by: - Removing manual post-provisioning
Ansible steps - Automating Tailscale setup via Cloud-Init - Updating
documentation to reflect new workflow

terraform/prod/variables.tf

@@ -14,6 +14,13 @@ variable "proxmox_url" {
 variable "proxmox_api_token" {
   description = "Token pour accéder à l'API Proxmox"
   type        = string
+  sensitive   = true
+}
+
+variable "tailscale_auth_key" {
+  description = "Clé d'authentification Tailscale pour enregistrer les noeuds."
+  type        = string
+  sensitive   = true
 }
 
 variable "admin_ssh_public_key_path" {
@@ -54,7 +61,6 @@ variable "virtual_machines" {
     network_model  = optional(string, "virtio")
     username       = optional(string)
     on_boot        = optional(bool, true)
-    dns_servers    = optional(list(string), ["8.8.8.8", "8.8.4.4"])
     tags           = optional(list(string), [])
 
     # Disques supplémentaires (optionnel)