This commit aims to replace static jobs by fully configurable build config.
Each build has a config (custom or inherited from repo or branch).
Each config has a list of steps.
For now, a step can test/run odoo or create a new child build. A python job is
also available.
The mimic the previous behaviour of runbot, a default config is available with
three steps, an install of base, an install+test of all modules, and a last step
for run.
Multibuilds are replace by a config containing cretaion steps.
The created builds are not displayed in main views, but are available
on parent build log page. The result of a parent takes the result of
all children into account.
This new mechanics will help to create some custom behaviours for specifics
use cases, and latter help to parallelise work.
