Config server fails to start if config hasn't been generated yet

```
$ CID=$(docker run -d --privileged -p 1194:1194/udp -p 443:443/tcp jpetazzo/openvpn)
$ docker run -t -i -p 8080:8080 --volumes-from $CID jpetazzo/openvpn serveconfig
Please run the OpenVPN container at least once in normal mode,
to generate the client configuration file. Thank you.
```
