Add the restructuredtext check to the flake8 job

This follows a recommendation in this thread on openstack-dev.
http://lists.openstack.org/pipermail/openstack-dev/2018-June/131233.html

Change-Id: I81f5283813a18a1807c1fe68b3fae83a35423c8c

tox.ini

@@ -11,7 +11,7 @@ setenv =
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        http://tarballs.openstack.org/horizon/horizon-master.tar.gz
-commands = 
+commands =
     find . -type f -name "*.pyc" -delete
     /bin/bash run_tests.sh -N --no-pep8 {posargs}
 whitelist_externals = find
@@ -21,7 +21,9 @@ setenv = DJANGO_SETTINGS_MODULE=mistraldashboard.test.settings
 
 [testenv:pep8]
 basepython = python3
-commands = flake8
+commands =
+    python setup.py check --restructuredtext --strict
+    flake8
 
 [testenv:venv]
 basepython = python3