Introduce the concept of "base services" (external services that
OpenStack components can assume will be present in any OpenStack
installation) and list current base services.
This is intended to reflect the current situation. Changes to the
list should be proposed as subsequent patches.
You can find a more detailed analysis and rationale for this change
in the Architecture WG analysis:
http://git.openstack.org/cgit/openstack/arch-wg/tree/active/base-services.rst
Change-Id: I77ffc000d895c3d8eb27b3b821c6c63f9dd2f675
59a483f8bd