[schema / sources] store data sources as cubicweb entities
this implies several changes:
* new CWSource / CWSourceHostConfig entity types
* only the system sources and default admin login/password stored in sources file
(other stuff will be ignored)
* on startup, get sources definition from the database
* every entities have a cw_source relation
* a facet allow filtering
[devtools] refactor http server initialization in a much saner way
Use a new ApptestConfiguration to group server paramater.
Fix base-url mangling between main site and data ressources.