fix Celery Result backend. DisabledBackend object has no attribute _g…

[gcdsss]

curl http://localhost:8004/tasks/<TASK_ID> will raise

AttributeError: 'DisabledBackend' object has no attribute '_get_task_meta_for'

root cause:

celery modules doesn't contain result_backend conf.

Fix:

Use celery instance in worker.py just to solve this problem