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:
- Specified correct instance of 'List'
This change also adds the "warning-is-error" setting
to setup.cfg in order to allow for strict doc validation
which will cause doc building to fail if any warnings
are thrown.
[0] http://lists.openstack.org/pipermail/openstack-dev/2017-March/113085.html
Change-Id: I8111193e5a1ae7d063ab4cc37186aea1299964a4