dashboard: add a last default value in grafana-server host section

dashboard: add a last default value in grafana-server host section

If there is no mgrs and mons in the inventory, it will fail with the following error:

```
ERROR! The field 'hosts' has an invalid value, which includes an undefined variable. The error was: 'dict object' has no attribute 'mons'

The error appears to be in '/home/guits/ceph-ansible/site-docker.yml.sample': line 539, column 3, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

- hosts: '{{ (groups["mgrs"] | default(groups["mons"]))[0] }}'
  ^ here
We could be wrong, but this one looks like it might be an issue with
missing quotes. Always quote template expression brackets when they
start a value. For instance:

    with_items:
      - {{ foo }}

Should be written as:

    with_items:
      - "{{ foo }}"
```

let's add an  `omit` so it just display this message instead:

```
PLAY [[]] *******************
skipping: no hosts matched
```

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