Install Python Docker library through pip

Summary:
Salt now uses Python 3, so the 2/3 logic isn't needed anymore.

EPEL 8 doesn't provide a package for this library. As such,
we fallback to an installation through pip.

Ref T1592.

Test Plan: Tested with import docker on Equatower

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1592

Differential Revision: https://devcentral.nasqueron.org/D2207