Provision Airflow DAGs folders

Summary:
Clone DAGs repositories and copy content to /srv/airflow/<realm>/dags,
using if needed a specific branch, revision or tag.

This state is intended to be called when new commit lands,
using Salt Reactor, as defined in D3280.

Ref T1812

Test Plan: salt dwellers state.apply roles/saas-airflow

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1812

Differential Revision: https://devcentral.nasqueron.org/D3281