Enhance tests for non-default default_domain_id

There were no tests that show what happens when the
default_domain_id is changed from the default. The new tests show
that in some cases the behavior is incorrect, and in other cases
it is correct.

Change-Id: Ia167e153f488617f94afce7fddfc52e1e2c8e79c
Related-Bug: #1265108
Remove unused member from KVS assignment

The KVS assignment backend's _get_user_by_name method was not used.
This change removes it.

Change-Id: I357adf745095b2ac868c3c6eb78e4ab6db2ca0fc
Merge "Enhance tests for non-default default_domain_id"
Merge "Remove unused member from KVS assignment"