Environment variables in server.cfg

I'd like to be able to reference environment variables in `server.cfg`. E.g.:

```
PARENT_PROXY:$PARENT_PROXY
PARENT_PROXY_PORT:$PARENT_PROXY_PORT
```

In my case, this configuration is used by both NTLMaps and another script. If NTLMaps supported this, it would enable me to remove the duplication.
