[airflow-cluster] Turn PYTHONHTTPSVERIFY=0 into airflow config and enable to option to import certificates

[hugowschneider]

This environment variable can be better handled as a airflow configuration which allows the value
to be set through environment variables and also through config files

https://github.com/opendevstack/ods-quickstarters/blob/7f8c371a14e3c1dd64c48dcbad404f3949b11350/airflow-cluster/base-images/airflow/dist/oauth/airflow_oauth/contrib/auth/backends/generic_oauth.py#L34-L36

Another option that must be included is the possibility to import the certificates into the pod using secrets