Treat sphinx warnings as errors
Treat sphinx warnings as errors

This enables sphinx to treat warnings as errors. With this change any
documentation format errors will be flagged in the CI sphinx job.

Change-Id: Ia0cc5a264f2c1db222156b655b706bc581f8820c

stackforge/senlin,stackforge/senlin,openstack/senlin,openstack/senlin,openstack/senlin