Webrunner tests and new tools for resiliency

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