Don't import Celery or Kubernetes at top level of CLI parser (#20675)
Don't import Celery or Kubernetes at top level of CLI parser (#20675)

The change to validate the subclassed executors mistakenly _required_ both celery
and kubernetes libraries installed to run `airflow`. Oopsie
Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,mistercrunch/airflow,mistercrunch/airflow,nathanielvarona/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,apache/airflow,lyft/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,apache/airflow,apache/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow