Celery execution fixes

Got this exception:

```
2014-12-04 20:00:50,853 INFO - Yet to run: 2 | Succeeded: 0 | Running: 0 | Failed: 0 | Won't run: 0
Process CelerySubmitter-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/home/paul_yang/Airflow/airflow/executors/celery_executor.py", line 51, in run
    key, command = self.task_queue.get()
TypeError: 'NoneType' object is not iterable
```