[AIRFLOW-5447] Don't set the executor with a default arg

[cwegrzyn]

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The call to get a default executor was being executed at module import time. If this module is imported by a DAG, it can lead to two executors running in the scheduler, which in the case of the KubernetesExecutor can cause a deadlock. This defers creating the default executor to initialization.

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[dimberman]

@cwegrzyn I made a PR against master with this fix and cherry-picked your change #6129