Define v3 identity_client as library

Migrate identity v3 API identity_client to tempest.lib and implement
unit test for the service clients.

Change-Id: If42e7aad0e57b1d6f8a17b8f6a646ac22a6b0fa3
diff --git a/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml b/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
index 6673616..d1635e3 100644
--- a/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
+++ b/releasenotes/notes/add-new-identity-clients-as-library-5f7ndha733nwdsn9.yaml
@@ -8,3 +8,4 @@
       * identity_client(v2)
       * groups_client(v3)
       * users_client(v3)
+      * identity_client(v3)