This change removes the unused "warnerrors" setting that
was part of [pbr] which was replaced by "warning-is-error"
in sphinx 1.5 and above[0]. This also fixes any warnings
and errors that came up when running `tox -edocs` using
this new feature:
- Invalid code example highlighting
- Redundant loading of todo extension
[0] http://lists.openstack.org/pipermail/openstack-dev/2017-March/113085.html
Change-Id: I9a8789b448ffa199b4539f57e692bac251d75036