[BUGFIX] added parameter to fetch categories flat

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | none
| Fixed tickets | none
| BC Breaks     | -
| Doc           | -

__Requires:__
- https://github.com/sulu-cmf/SuluCategoryBundle/pull/18