fix Hetzner group names (kubernetes-sigs#11232)

The inventory file generated by Terraform produces the following warnings:
```
[WARNING]:  * Failed to parse <PATH>/kubespray/contrib/terraform/hetzner/inventory.ini with ini plugin:
<PATH>/kubespray/contrib/terraform/hetzner/inventory.ini:21: Section [k8s_cluster:children] includes undefined group: kube-master
...
[WARNING]: Could not match supplied host pattern, ignoring: kube-master

PLAY [Add kube-master nodes to kube_control_plane] ********************************************************************************************************
skipping: no hosts matched
[WARNING]: Could not match supplied host pattern, ignoring: kube-node

PLAY [Add kube-node nodes to kube_node] *******************************************************************************************************************
skipping: no hosts matched
```

contrib/terraform/hetzner/templates/inventory.tpl

@@ -12,8 +12,8 @@ ${list_master}
 ${list_worker}
 
 [k8s_cluster:children]
-kube-master
-kube-node
+kube_control_plane
+kube_node
 
 [k8s_cluster:vars]
 network_id=${network_id}