[AIRFLOW-2875] Escape env vars in tmp config

[wdhorton]

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  When writing tmp config files, escapes the env variables, since they are read by ConfigParser when the tmp config file is loaded when the task is being run.

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[ashb]

What if someone is using % interpolation in their config file? Have they already been replaced by the stage we run the replace?

[wdhorton]

This doesn't affect values that are in the config file, just ones that are being pulled from the env (like AIRFLOW__CELERY__BROKER_URL in my case) and written to the tmp config file with tmp_configuration_copy(). So % interpolation in the config file should continue to work as is.

That said, it looks like I broke a bunch of tests, so I need to figure out how this is having an effect in other places that I didn't expect.

[ashb]

Closed in favour of #4029 (sorry for missing this!)