Identity v3 Domain Configuration Client

Adds a new client lib to handle the domain configuration API.

The domain configuration API is part of the standard
keystone v3 API [0].

This patch also adds unit tests and API tests for the domain
configuration client. Most of its APIs are RULE_ADMIN_REQUIRED
[1] and so the API tests are included in the admin namespace.

[0] https://developer.openstack.org/api-ref/identity/v3/
[1] https://github.com/openstack/keystone/blob/master/keystone/common/policies/domain_config.py

Change-Id: I2f6229076aa7d2939dd91c487085ea73de001403
diff --git a/releasenotes/notes/add-domain-configuration-client-tempest-tests-e383efabdbb9ad03.yaml b/releasenotes/notes/add-domain-configuration-client-tempest-tests-e383efabdbb9ad03.yaml
new file mode 100644
index 0000000..5653681
--- /dev/null
+++ b/releasenotes/notes/add-domain-configuration-client-tempest-tests-e383efabdbb9ad03.yaml
@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Add a new client to handle the domain configuration feature from the
+    identity v3 API.