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: I861b6de50aaa5a30db1b18474b2554743a9968c7

mandeepdhami/stevedore,openstack/stevedore,mandeepdhami/stevedore