Implement a test that validates the user_configuration

Some commits may break the user_configuration of some workers. However, the developer won't notice it till he sees the Git import process on Arkindex fail miserably (if he has access to it). That's wasted on time on the dev and on the Arkindex agent.

We could create a new test that makes sure that the worker_configuration is valid. This test would then be ported into

• the cookiecutter worker
• most workers via PRs