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

setup.cfg

@@ -16,5 +16,8 @@ all_files = 1
 build-dir = doc/build
 source-dir = doc/source
 
+[pbr]
+warnerrors = True
+
 [wheel]
 universal = 1

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 minversion = 1.6
+envlist = docs
 skipsdist = True
 
 [testenv]
-sitepackages = True
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
@@ -12,5 +12,5 @@ deps = -r{toxinidir}/requirements.txt
 [testenv:venv]
 commands = {posargs}
 
-[testenv:build_specs]
+[testenv:docs]
 commands = python setup.py build_sphinx