overlap pod should only be checked on running pods

[penfree]

now, KubernetesPodOperator will check if there is an existing pod of this task, if the pod matches, it can be reused. But is does not filter completed or failed pods. when the task failed, it cannot be retried because there is already one failed pod with the label.

[2020-08-19 23:14:56,770] {{taskinstance.py:1150}} ERROR - Pod Launching failed: More than one pod running with labels: dag_id=IncrementDataBuild_wjwjyrmyy,execution_date=2020-08-19T164816.2350170000-ac196c6dc,task_id=exportdata-wjwjyrmyy
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/airflow/contrib/operators/kubernetes_pod_operator.py", line 276, in execute
    '{label_selector}'.format(label_selector=label_selector))
airflow.exceptions.AirflowException: More than one pod running with labels: dag_id=build,execution_date=2020-08-19T164816.2350170000-ac196c6dc,task_id=exportdata

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
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[kaxil]

Can you rebase your PR on latest master please

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.