Enable warnerrors for pbr to catch Sphinx warnings

With this change, we now should properly gate on Sphinx errors /
warnings.

Change-Id: I7e789ad83e81286575c74e1e5256da20a717e982
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
diff --git a/setup.cfg b/setup.cfg
index caa15d6..242a2c3 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -16,6 +16,9 @@
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 2.6
 
+[pbr]
+warnerrors = True
+
 [global]
 setup-hooks =
     pbr.hooks.setup_hook