Check RST as part of linting

Check our RST files so that uploading to PyPI works fine with a linter
check.

Change-Id: I41cb71409ea5cb7d1e6d8b546a37633ee481a38d

tox.ini

@@ -16,6 +16,7 @@ commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
 commands =
+    python setup.py check --restructuredtext --strict
     flake8
     # Run doc8 to check .rst and .txt files.
     # HACKING.rst is the only file that is not referenced from