only install things in the venv for pep8

we can use tox to run tempest, but to act like the gate we
shouldn't install a venv for full or smoke runs.

only do this for pep8, as we need hacking installed from pip

Change-Id: I02a62d3f0ae9759e619976e523f75c64a9af577d
Merge "only install things in the venv for pep8"