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 warnerrors one.

There are a couple of small items to be cleaned up.

Change-Id: I01b1ac49c69d001ea25a475d903311d0b592656e
diff --git a/setup.cfg b/setup.cfg
index 96313fd..b2035bc 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -45,12 +45,10 @@
     tempest.config = tempest.config:list_opts
 
 [build_sphinx]
-all_files = 1
+all-files = 1
+warning-is-error = 1
 build-dir = doc/build
 source-dir = doc/source
 
-[pbr]
-warnerrors = True
-
 [wheel]
 universal = 1