Support Sphinx >=1.3 new protoype and warnings

Sphinx >= 1.3 changes:
- Prototypes for init_values() requires a handle to warnings.warn
- New warning for setting the html_theme in conf.py to 'default':
   sphinx.errors.SphinxWarning: WARNING: 'default' html theme has been
   renamed to 'classic'. Please change your html_theme setting either
   to the new 'alabaster' default theme, or to 'classic' to keep using
   the old default.

This change deals with the above sphinx updates by adding the handle to
init_values() and setting pbr to _not_ fail on warnings when building the
docs.

Change-Id: Iba92628263e20efca84aeada0e19000f4c9b1fac

pbr/builddoc.py

@@ -17,7 +17,9 @@
 from distutils import log
 import fnmatch
 import os
+import pkg_resources
 import sys
+import warnings
 
 try:
     import cStringIO
@@ -130,14 +132,19 @@ class LocalBuildDoc(setup_command.BuildDoc):
         if self.today:
             confoverrides['today'] = self.today
         sphinx_config = config.Config(self.config_dir, 'conf.py', {}, [])
-        sphinx_config.init_values()
+        sphinx_ver = pkg_resources.get_distribution("sphinx").version
+        if pkg_resources.parse_version(sphinx_ver) >= \
+                pkg_resources.parse_version('1.3.1'):
+            sphinx_config.init_values(warnings.warn)
+        else:
+            sphinx_config.init_values()
         if self.builder == 'man' and len(sphinx_config.man_pages) == 0:
             return
         app = application.Sphinx(
             self.source_dir, self.config_dir,
             self.builder_target_dir, self.doctree_dir,
             self.builder, confoverrides, status_stream,
-            freshenv=self.fresh_env, warningiserror=True)
+            freshenv=self.fresh_env, warningiserror=False)
 
         try:
             app.build(force_all=self.all_files)