Add the ability to define server settings from a python module

Today the settings resolution order is the following:

1. Core default setting
2. Packages settings
3. YAML config file

Developpers might want to keep the ability to execute code when defining new parameters (as django allows it).

Advice from @calummackervoy would be to add an extra step beteen 2 and 3 to load parameters from a settings.py file.