Add doc8 tox env

doc8 is the pep8 for docs

Change-Id: I7eb77c90d158b8cfad2c8c3439e13fac4a576343

tox.ini

@@ -32,6 +32,13 @@ downloadcache = ~/cache/pip
 sitepackages = False
 commands = flake8
 
+[testenv:doc8]
+deps =
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/test-requirements.txt
+    doc8
+commands = doc8 doc/source
+
 [testenv:venv]
 commands = {posargs}