Create workspace directory for Airflow pipelines

Summary:
The FANTOIR pipeline requires a working directory. The directory ~/workspace
is mounted in every Airflow container, including airflow <realm> shell
for maintenance purpose.

Ref T1812.

Test Plan: Use that directory with D2754.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1812

Differential Revision: https://devcentral.nasqueron.org/D3276