Using config_template

Using config_template

Thanks to @cloudnull great patch at
https://github.com/ansible/ansible/pull/12555
we now have the ability to add more configuration options instead of
having to push a PR to add a new option to the template. So you can
dynamically add and remove flags.

To use it, edit `ceph_conf_overrides` in `group_vars/all` like so:

```
ceph_conf_overrides
 global:
     foo: 12345
     bar: 6789
```

Signed-off-by: Sébastien Han <seb@redhat.com>