docs - Set pbr 'warnerrors' option for doc build
docs - Set pbr 'warnerrors' option for doc build

By setting this pbr option in setup.cfg, the doc build will fail in case
of any warnings or errors occur during the build process.

Change-Id: Ic3a4b0a641a92e9fabde2a871eed478ba99a25e1

openstack/automaton