API: support for the webapp

[matzf]

The scion-apps webapp (to be renamed to ... netsec-ethz/scion-apps#108 ?), is an application that can be run by SCIONLab users on the VM running their AS.
For it's functionality, it requires:

• AS labels
• AS geo coordinates
• list of bandwidth test servers

This information is currently scraped from various ancient sources.
Additionally, some API keys (and certain URLs to scrape the above information from), are retrieved from a tiny central web service (see netsec-ethz/scion-apps#146), to avoid storing them in clear text in the public webapp source code and to allow centralized updates to the keys.

Add APIs to support retrieving this information from scionlab.org directly.

[nicorusti]

This API would come in very handy in order to keep the node map on scion-architecture.net up to date.
(As discussed on Element)