Enhance auth tests for non-default default_domain_id

There wasn't an authentication test that showed what happens when
the default_domain_id option is changed from the default. The new
tests show that the behavior is incorrect.

This test is split out because it uses a new feature of the KVS
backend to allow adding users with the same name in different
domains, because I don't think we'll backport that change.

Change-Id: I890a1c798d7f4a8a345404cd7754708ab0d36651
Related-Bug: #1265108
Merge "Enhance auth tests for non-default default_domain_id"