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/README.md

@@ -58,7 +58,6 @@ Chaque VM peut avoir des configurations optionnelles :
   datastore             = "local-lvm"      # Datastore Proxmox
   network_bridge        = "vmbr0"         # Bridge réseau
   username              = "admin"         # Utilisateur (défaut: damien)
-  dns_servers           = ["1.1.1.1"]     # Serveurs DNS
   post_install_commands = ["apt update"]  # Commandes post-installation
 
   # Disques supplémentaires