Make pep8 *the* linting interface

According to the PTI (=Python Test Interface,
http://governance.openstack.org/reference/cti/python_cti.html), pep8
is the interface for codestyle checks. Move all tests from linters to
pep8.

This change will be followed by a change to project-config to use pep8
for testing in the gate.

Change-Id: I1a48f4b1e0b0950640192bcfc55121619a844b50