Install docker-py for Docker Salt minions

Summary:
salt.modules.dockerng requires docker-py, the legacy package for
docker, a Python library for the Docker Engine API.

Fixes T1359.

References:

• https://docs.saltstack.com/en/latest/ref/modules/all/salt.modules.dockerng.html#installation-prerequisites
• https://pypi.python.org/pypi/docker-py

Test Plan: Deploy on Dwellers and Equatower

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1359

Differential Revision: https://devcentral.nasqueron.org/D1407