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
KVS support domain as namespace for users

The KVS identity backend didn't support having users with the
same name in different domains.

Change-Id: I4f6a9010ed085aef9cba5f368e292dd28339a0fa
Related-bug: #1265108
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
Fix using non-default default_domain_id

Changing the default_domain_id to something other than the
default would cause several operations to not work correctly.
This is because the value of default_domain_id was read and
cached in several parts on import, which is before the call to
CONF() so the cached value is the original default ('default')
rather than the value that the operator has in the config file.

This change removes the cached default_domain_id and instead
reads the value when it's needed at runtime so that the
user-configured value is used instead of the original default.

Change-Id: I9543e2108866ed66000d75f868bbd2b1ed978b97
Closes-Bug: #1265108
Merge "Enhance tests for non-default default_domain_id"
Merge "Remove unused member from KVS assignment"
Merge "KVS support domain as namespace for users"
Merge "Enhance auth tests for non-default default_domain_id"
Merge "Fix using non-default default_domain_id"