Implement domain member functionality for user API
Implement domain member functionality for user API

This commit adds explicit testing for how users with the member role
on a domain should interact with users both inside and outside of
the domain they have authroization on.

Subsequent patches will continue to build on this by incorporating:

 - domain admin functionality
 - project user test coverage

Change-Id: I2a75f49e953c0c6fbe2de6da7b40faaa208f953d
Partial-Bug: 1748027
Partial-Bug: 968696

mahak/keystone,mahak/keystone,openstack/keystone,openstack/keystone,openstack/keystone,mahak/keystone