Optionally disable ZooKeeper logging

Optionally disable the output from the underlying `go-zookeeper` library. E.g.:

```
2018/01/09 13:09:35 Connected to [::1]:2181
2018/01/09 13:09:35 Authenticated: id=99320884360249348, timeout=10000
2018/01/09 13:09:35 Re-submitting `0` credentials after reconnect
```