nixos/graylog: fix startup

Until now the startup failed with an error like this:

```
com.github.joschi.jadconfig.ValidationException: Parent path /var/lib/graylog/server for Node ID file at /var/lib/graylog/server/node-id is not a directory
```

This happens since `graylog.service` ensures that `/var/lib/graylog`
exists, however it doesn't take care of the directory for
`cfg.nodeIdFile`.