Need to be able to configured external and internal URLs, so the app server can communicate through private network and users through public network.

* For internal connection info, I think we can assume HTTP through the private network, no HTTPS needed on etherpad as the web proxy can terminate SSL. So the internal hosts would just be an array of host+port pairs
* For external connection info, we can probably have a single config for protocol (optional). Though this config would be replaced when https://github.com/sakaiproject/Hilary/issues/473 is completed, and we can just use that.