containers: fix bug when looking for existing cluster
containers: fix bug when looking for existing cluster

When containerized deployment, `docker_exec_cmd` is not set before the
task which try to retrieve the current fsid is played, it means it
considers there is no existing fsid and try to generate a new one.

Typical error:

```
ok: [mon0 -> mon0] => {
    "changed": false,
    "cmd": [
        "ceph",
        "--connect-timeout",
        "3",
        "--cluster",
        "test",
        "fsid"
    ],
    "delta": "0:00:00.179909",
    "end": "2018-01-09 10:36:58.759846",
    "failed": false,
    "failed_when_result": false,
    "rc": 1,
    "start": "2018-01-09 10:36:58.579937"
}

STDERR:

Error initializing cluster client: Error('error calling conf_read_file: errno EINVAL',)
```

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,font/ceph-ansible,font/ceph-ansible,fgal/ceph-ansible,travmi/ceph-ansible,ceph/ceph-ansible,travmi/ceph-ansible,fgal/ceph-ansible