Mirror tox and sphinx fixes from nova-specs repo

This commit copies some changes from the nova-specs repo. This fixes
a couple of issues around the tox.ini file and the setup.cfg to make
running a docs build infra job work well and catch formatting
failures.

The nova-specs commits this is based on:

4a6cb70 Make sphinxbuild fail on formatting errors
775d690 Allow running 'tox' with no arguments
8947230 Disable site-packages for tox

Change-Id: If416c68e9a608039e74cadb2d50915fa6b859d89