Allow Airflow to connect to Vault

Summary:
Airflow configuration is moved to airflow.cfg, that will be easier to read
and maintain than environment variables, especially for Vault back-end.
Also, that allows a faster deployment time as secrets need only to be read once.

The Vault application role matches the airflow policy as defined in D3270.

Ref T1812.

Test Plan:

• redeploy Airflow containers on Dwellers
• airflow config get-value secrets backend
• airflow connections get postgresql_fantoir

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1812

Differential Revision: https://devcentral.nasqueron.org/D3273