Removing the need of importing generator functions

[hug-dev]

In the beginning of generator.py, each generator function needs to be imported manually. With some clever Python importing mechanisms, maybe using wildcard (*) at some places, it might be possible to import all generator functions by default.
That way, adding a new test case ("toto") in the mock service would only consist in:

• adding the toto.spec.yaml file
• creating the generator function file toto.py containing the gen function