Use Sphinx 1.5 warning-is-error

This will ensure doc warnings don't make their way in.

Change-Id: I9a007ad89f3a2219feb960fee858bf70c1643416

Author: stephenfin

setup.cfg

@@ -696,9 +696,10 @@ autodoc_tree_excludes =
     openstackclient/tests/*
 
 [build_sphinx]
+all-files = 1
+warning-is-error = 1
 source-dir = doc/source
 build-dir = doc/build
-all_files = 1
 
 [upload_sphinx]
 upload-dir = doc/build/html