site-docker: fix undefined variable error
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 <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,fgal/ceph-ansible,fgal/ceph-ansible,ceph/ceph-ansible