OpenStack themed sphinx extension

This is about extracting the OpenStack theme from the various projects so we only need to have one copy of it. We may also move the code that auto-generates the API doc stub files into the library, since that is also copied into several projects.

The new library will be called oslo.sphinx and its API will be used by adding "oslo.sphinx" to the list of extensions in the sphinx conf.py file for the doc project.

There is one template, with a couple of blocks that can be overridden (those need to be documented).

There is no importable code, yet, and we don't anticipate any.

There will be a configuration setting to turn the API stub generation on and off, when that code is brought in.