Adds tests for the Webrunner, and updates its code to make it easier to test. These tests are still incomplete, but they have the merit of existing. I don't really like mocking function calls, but globally it's a better way than spawning a supervisord server and recreating the whole slaprunner's system tree.  

Also adds tools (= scripts) which will be usefull for the monitoring in slapos : 
  * generation of RSS feed from status items, and management of them.
  * script for creating promises based on RSS feeds 

/reviewed-on nexedi/slapos.toolbox!8

generatefeed: new helper script to easily create RSS2 Feeds from a bunch of well-formated JSON files

This first commit adds a script which generate a RSS feed from the statistics from rdiff-backup

It makes tests easiers and lighten the code in view.py

The code in the view is lighter, and the generic part could have been moved to utils

Doing so allows more grain control over the tests, as sup_process is just a
syntax sugar over RPC calls.

test / runner: new test for runSoftwareWithLock and runInstanceWithLock, which are both base functions of the webrunner

This also makes the function easier to test

  [agent] referer to an extra_requires which don't exist anymore

  Use json as configuration format
  Accept remote configuration file (url to a json)
  Introduce API to assert reponse codes and result for http and dns
  Add Unit tests.