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

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>
(cherry picked from commit 51cf3b7fa0211fbbfcbd8c4228dcd39d20f02e54)
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>