Enable warning-is-error in doc build

pbr warnerrors is no longer needed.

Change-Id: I0268962ab4856e8234a7218f961dfb9b2a59ee0d

setup.cfg

@@ -31,13 +31,11 @@ fixtures =
   mock>=2.0 # BSD
   fixtures>=3.0.0 # Apache-2.0/BSD
 
-[pbr]
-warnerrors = true
-
 [build_sphinx]
 source-dir = doc/source
 build-dir = doc/build
 all_files = 1
+warning-is-error = 1
 
 [upload_sphinx]
 upload-dir = doc/build/html