add flatcar support for Hetzner (#9618)

contrib/terraform/hetzner/modules/kubernetes-cluster-flatcar/templates/machine.yaml.tmpl

@@ -0,0 +1,16 @@
+---
+passwd:
+  users:
+    - name: ${user_flatcar}
+      ssh_authorized_keys: ${ssh_keys}
+storage:
+  files:
+    - path: /home/core/works
+      filesystem: root
+      mode: 0755
+      contents:
+        inline: |
+          #!/bin/bash
+          set -euo pipefail
+          hostname="$(hostname)"
+          echo My name is ${name} and the hostname is $${hostname}