Ignore doc build path

If you build the docs with `tox -edocs` and then run `tox -epep8`,
it will always fail because it checks the build output from the
docs. This change ignores the build folder.

Trivial-Fix
Change-Id: Ie7c53b517b7cfff1d317cad014371c94c6212eac

tox.ini

@@ -42,7 +42,7 @@ deps =
 commands =
   {toxinidir}/tools/run-bashate.sh
   flake8 {posargs}
-  doc8 doc
+  doc8 doc/source
   python {toxinidir}/tools/validate-all-file.py
   bandit --skip B303 -r ansible kolla_ansible tests tools
   yamllint -s .