Add doc8 tox env

doc8 is the pep8 for docs

Change-Id: I5f5b1bd02cb5d3702c59d2b2dd89bb32845a6968

tox.ini

@@ -38,6 +38,13 @@ commands =
     # Check that .po and .pot files are valid:
     bash -c "find sahara -type f -regex '.*\.pot?' -print0|xargs -0 -n 1 msgfmt --check-format -o /dev/null"
 
+[testenv:doc8]
+deps =
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/test-requirements.txt
+    doc8
+commands = doc8 doc/source
+
 [testenv:venv]
 commands = {posargs}