Tox entry point to run flake8

Tox is a thin wrapper around python virtualenv. It lets one define entry
points that run whatever commands the devs want, Jenkins just invoke a
list of known entry point.  That makes it easier for developers to
adjust what Jenkins is going to run.

Change-Id: I16c92cb7e40cefb5b87e0e3522fa72f67cc3ca79