site-docker: fix undefined variable error

`mon_group_name` isn't defined here, we must hardcode it.

Typical error:

```
The task includes an option with an undefined variable. The error was: 'mon_group_name' is undefined
```

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

site-docker.yml.sample

@@ -363,7 +363,7 @@
   become: True
   tasks:
     - name: get ceph status from the first monitor
-      command: docker exec ceph-mon-{{ hostvars[groups[mon_group_name][0]]['ansible_hostname'] }} ceph --cluster {{ cluster }} -s
+      command: docker exec ceph-mon-{{ hostvars[groups['mons'][0]]['ansible_hostname'] }} ceph --cluster {{ cluster }} -s
       register: ceph_status
       changed_when: false
       delegate_to: "{{ groups['mons'][0] }}"