Use Sphinx 1.5 warning-is-error

With pbr 2.0 and Sphinx 1.5, the setting for treat sphinx warnings as
errors is setting warning-is-error in build_sphinx section. Migrate the
setting from the old warnerror one.

Change-Id: Icd5cf4c279c566e1eddeede8df11a9d1c4d4c83f

Author: ajaeger

setup.cfg

@@ -188,6 +188,7 @@ nova.scheduler.driver =
 all_files = 1
 build-dir = doc/build
 source-dir = doc/source
+warning-is-error = 1
 
 [build_apiguide]
 all_files = 1
@@ -219,8 +220,3 @@ universal = 1
 [extras]
 osprofiler =
   osprofiler>=1.4.0 # Apache-2.0
-
-[pbr]
-# Treat sphinx warnings as errors during the docs build; this helps us keep
-# the documentation clean.
-warnerrors = true