Allow to enter a shell for Airflow maintenance operations

Summary:
To ease debug and testing, a shell with python is convenient.

Also to run commands in batch, like to drop default user and create
correct users account, a shell is faster than spin new containers
each time.

This change also correctly pass arguments to airflow command when a shell
is not needed, so airflow nasqueron users delete --username root works.

Ref T1812

Test Plan: airflow shell

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1812

Differential Revision: https://devcentral.nasqueron.org/D3275