[AIRFLOW-1587] Fix CeleryExecutor import error
[AIRFLOW-1587] Fix CeleryExecutor import error

`CeleryExcutor` can not import directly from
`airflow.executors`, we should import from
`airlfow.executors.celery_executor` instead.

Closes #2590 from
lxneng/bugfix/fix_pkg_importerror_in_run_task_view

mrares/incubator-airflow,mrares/incubator-airflow,mrares/incubator-airflow,mrares/incubator-airflow