sphinx is now stricter

The documentation files often have minor typo or badly formatted
commands which makes sphinx emits a warning. To make sure we always
catch them via the Jenkins build, this patch makes sphinx to convert
warning to errors thus aborting the build, simply add to pass the -W
option to sphinx-build.

This patch also fix one warning:
* There is no source/_static dir so disable html_static_path

Test plan:

$ make clean; make html
rm -rf build/*
sphinx-build -b html -d build/doctrees  -W source build/html
Making output directory...
Running Sphinx v1.1.3
loading pickled environment... not yet created
building [html]: targets for 18 source files that are out of date
updating environment: 18 added, 0 changed, 0 removed
reading sources... [100%] zuul
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] zuul
writing additional files... genindex py-modindex search
copying static files... done
dumping search index... done
dumping object inventory... done
build succeeded.

Build finished. The HTML pages are in build/html.
$

Change-Id: I55935fb5e869919ac639f841876aaf3871c71d17
Reviewed-on: https://review.openstack.org/16328
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins