Simplify zones/cells by making KeyStone service catalogs better.

At their most abstract, zones are a meta-concept layered over independent installations of OpenStack services. Whether nova cells, or swift zones, quantum zones, or federated instances of glance, the base zone need not be aware of the infrastructure, data, or implementation of their peers. After authenticating with Keystone, you receive back a service catalog containing the end-points for each service. Since endpoints is a list, we already expose a way of handling zones in a simple, effective manner.