library: Fix new-style modules check mode

library: Fix new-style modules check mode

Running the `ceph_crush.py`, `ceph_key.py` or `ceph_volume.py` modules in check
mode resulted in the following error:

```
New-style module did not handle its own exit
```

This was due to the fact that they simply returned a `dict` in that case,
instead of calling `module.exit_json()`.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>