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

cloud-init/user-data.yml.tftpl

@@ -0,0 +1,29 @@
+#cloud-config
+
+# 1. Configuration du nom d'hôte de la machine
+hostname: ${hostname}
+fqdn: ${hostname}
+preserve_hostname: false
+
+# 2. Création de l'utilisateur
+users:
+  - name: ${username}
+    sudo: ALL=(ALL) NOPASSWD:ALL
+    groups: sudo, admin
+    shell: /bin/bash
+    ssh_authorized_keys:
+      - ${ssh_key}
+
+# 3. Mise à jour du système
+package_update: true
+package_upgrade: true
+
+# 4. Installation des paquets
+packages:
+  - qemu-guest-agent
+  - curl
+
+# 5. Installation et configuration de Tailscale
+runcmd:
+  - [ sh, -c, "curl -fsSL https://tailscale.com/install.sh | sh" ]
+  - [ tailscale, up, --authkey=${tailscale_auth_key}, --hostname=${vm_name} ]