Add .rst files check to pep8

Errors in README.rst prevent the package from being registered in Pypi. We would like pep8 to fail in these cases.
For more details, see: http://lists.openstack.org/pipermail/openstack-dev/2018-June/131233.html

Change-Id: Ic6f92abd09740372a829b9a58902e1c8f63b5bcc

doc/requirements.txt

@@ -5,3 +5,4 @@ oslo.config>=5.2.0 # Apache-2.0
 openstackdocstheme>=1.18.1 # Apache-2.0
 sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
 reno>=2.7.0 # Apache-2.0
+docutils>=0.11  # OSI-Approved Open Source, Public Domain

lower-constraints.txt

@@ -140,3 +140,4 @@ WebTest==2.0.29
 Werkzeug==0.14.1
 wrapt==1.10.11
 futures==3.0.0
+docutils==0.11

tox.ini

@@ -21,7 +21,12 @@ passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
 basepython = python3
+deps = -r{toxinidir}/doc/requirements.txt
+       -c{toxinidir}/lower-constraints.txt
+       -r{toxinidir}/test-requirements.txt
+       -r{toxinidir}/requirements.txt
 commands = flake8
+           python setup.py check --restructuredtext --strict
 
 [testenv:genconfig]
 basepython = python3