Add separate project for training-guides

The OpenStack Training Guides team has been sheltering under
the loving arms of the openstack-manuals repo for the recent past.

It's now time to move them out to further their independent work.

This change adds a project:

openstack/training-guides

seeded with the training-guides subdirectory from openstack-manuals,
and attempts to configure the jenkins/zuul jobs so that the existing
docbook check, gate and post will also run on the new project.

Change-Id: I1aed9efa8ca3fd106872cec248cd7c4964fa9adf
Merge "Add separate project for training-guides"