Summary:
Needs to be enabled by option PYTHON_VENV in the config.

shell_builder.py sets up the venv and uses it once; calling activate

For docker we set ENV; resulting in the virtual environment being present
when the resulting container is run as well as at build time. This is also cleaner
and easier to follow than re-asserting on each RUN step.

For Lego builder we need to source activate on each command as environment
will not persist between commands.

While man on the posts say it makes no sense to use virtualenv within docker
container, this method simplifies the process considerably as we can rely on the
name pip being valid and we don't need to either ensure we are root or pass the
--user flag to pip and setuptools.

Pull Request resolved: https://github.com/facebookincubator/LogDevice/pull/76

Reviewed By: wez

Differential Revision: D14875633

Pulled By: calebmarchent

fbshipit-source-id: aabbcdd509d2a59fa36f8004032a052f014ce1ba