FIX: Bastion undefined variable (kubernetes-sigs#7227)

Fixes the following error when using Bastion Node with the sample config.
```
fatal: [bastion]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute 'bastion'\n\nThe error appears to be in '/home/felix/inovex/kubespray/roles/bastion-ssh-config/tasks/main.yml': line 2, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n- name: set bastion host IP\n  ^ here\n"}
```

inventory/sample/inventory.ini

@@ -10,6 +10,7 @@
 # node6 ansible_host=95.54.0.17  # ip=10.3.0.6 etcd_member_name=etcd6
 
 # ## configure a bastion host if your nodes are not directly reachable
+# [bastion]
 # bastion ansible_host=x.x.x.x ansible_user=some_user
 
 [kube-master]