Fix PEP8 tox environment
Fix PEP8 tox environment

It has checks for documentation however it was missing the documentation
requirements.

This patch adds doc/requirements.txt as a source for pip packages.

Change-Id: Iba157100923d26ff42f78378f2fe4357a183673e

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl