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. Migrate
the setting from the old warnerrors one.

There are a couple of small items to be cleaned up.

Change-Id: I01b1ac49c69d001ea25a475d903311d0b592656e
3 files changed