Isolate configuration related code better

- create the global `config` object inside the
  `urban_meal_delivery.configuration` module
- streamline documentation and comments

tests/conftest.py

@@ -5,6 +5,8 @@ import os
 from urban_meal_delivery import config
 
 
+# The TESTING environment variable is set
+# in setup.cfg in pytest's config section.
 if not os.getenv('TESTING'):
     raise RuntimeError('Tests must be executed with TESTING set in the environment')