client: try to kill dummy container only on first client node

The 'dummy' container is created only on first client node, it means we
must seek to destroy this container only on this node, otherwise this
can cause failure like following :
```
fatal: [192.168.24.8]: FAILED! => {"changed": false, "cmd": ["docker", "rm",
"-f", "ceph-create-keys"], "delta": "0:00:00.023692", "end": "2018-06-12
20:56:07.261278", "msg": "non-zero return code", "rc": 1, "start":
"2018-06-12 20:56:07.237586", "stderr": "Error response from daemon: No such
container: ceph-create-keys", "stderr_lines": ["Error response from daemon: No
such container: ceph-create-keys"], "stdout": "", "stdout_lines": []}

```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1590746

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

roles/ceph-client/tasks/create_users_keys.yml

@@ -109,8 +109,9 @@
 - name: kill a dummy container that created pool(s)/key(s)
   command: docker rm -f ceph-create-keys
   changed_when: false
-  run_once: true
-  when: containerized_deployment
+  when:
+    - containerized_deployment
+    - inventory_hostname == groups.get(client_group_name) | first
 
 - name: get client cephx keys
   copy: