Split users_client from keystone client v2.

Removes users client from identity client v2 into an own one.

Partially implements blueprint consistent-service-method-names

Change-Id: I2473fc29c68c763d02fe2d6ce59266e0eb84816d
diff --git a/tempest/stress/driver.py b/tempest/stress/driver.py
index a3e1ea3..6531059 100644
--- a/tempest/stress/driver.py
+++ b/tempest/stress/driver.py
@@ -148,12 +148,15 @@
                     identity_client = admin_manager.identity_client
                     projects_client = admin_manager.tenants_client
                     roles_client = admin_manager.roles_client
+                    users_client = admin_manager.users_client
                 else:
                     identity_client = admin_manager.identity_v3_client
                     projects_client = None
                     roles_client = None
+                    users_client = None
                 credentials_client = cred_client.get_creds_client(
-                    identity_client, projects_client, roles_client)
+                    identity_client, projects_client, roles_client,
+                    users_client)
                 project = credentials_client.create_project(
                     name=tenant_name, description=tenant_name)
                 user = credentials_client.create_user(username, password,