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: I1f70ea5c6b26b30d5c5583934333053ef065fbf6

openstack/taskflow,pombredanne/taskflow-1,openstack/taskflow,jimbobhickville/taskflow,jimbobhickville/taskflow,pombredanne/taskflow-1