Allow specification of optional domain name space

This documents the v3 api changes proposed to represent the optional
support for domain name spaces, allowing uniqueness of user and
project names to be either global or domain wide.  These changes
match the spec that was part of the blueprint.

blueprint domain-name-spaces

DocImpact

Change-Id: Ib824c5c9f441a78b5009e1954d97427e869ed2e4