Remove pbr setup hook from setup.cfg

The setup hook is no longer necessary since we don't use d2to1.

Change-Id: I3ece334c0096ae3a17bf4f350501a71ef7ffa422
Use the python in the virtualenv for everything

Upgrade the tox requirements to match the rest of OpenStack. Benefits
include not making an sdist first, and ensuring that the virtualenv is
use for all actions.

(Mirrors Tuskar patch: https://review.openstack.org/#/c/50091/)

Change-Id: Ibcefee6520b2c1289b8dfe48d49d8ead298a5a00