This commit fixes all warnings created by flake8 by removing unused
imports and variable assignments. Some wrongly indented lines are now
indented correctly.
- don't execute celery tasks directly
- introduce optional parameters to fileserver tasks to allow chaining
- handle user/group/key create and delete tasks in new osusers.signals
class
- adapt unit tests
- change TaskResults model to store the task signatures
- generalize the local settings' logging configuration
