Add doc venv to align with other OpenStack projejcts (such as keystone,
heat and nova-specs) who are using 'docs' for there doc builds. Also
enable warnerrors in pbr to raise an error if there are any docs
formatting warnings.
Change-Id: Ifa31edf3c3392106a8aa1352bdd557bde2f1b9a1
da0a33a85a