Use a script to build and copy RST guides around instead of duplicating
the same lines every time.
Use script in build-user-guides.sh for now. A follow-up patch shall
change further places.
This script will eventually be moved over to openstack-doc-tools, let's
first evolve it here and check that we cover all use cases.
Change-Id: I753e742bf77b2d6f5acf2f909d62af3dd422832a
94961a3c3a