Revert "Use python -m to call virtualenv in init_venv."

This reverts commit 40781b163e1f11f62f0df20f9dec16b3e06e8daf.

Some platforms (e.g. Ubuntu Trusty) don't have a python3-virtualenv, so we
should instead adjust the documentation to use the python2 one.
INSTALL: Use python2 virtualenv for Debian.