Use Sphinx 1.5 warning-is-error
Use Sphinx 1.5 warning-is-error

With pbr 2.0 and Sphinx 1.5, the setting for treat sphinx warnings as
errors is setting warning-is-error in build_sphinx section. Enable this

Change-Id: I3712c681cedae30c189d975f6b319e278c1bd419

openstack/python-heatclient