Get credential IDs from Credentials class

With the move to Credentials class, tenant_id and user_id are
available in the Credentials instance.
Several test use identity admin client to obtain tenant_id
and user_id, fixing them to get the IDs from Credentials instead.

Partially implements: bp multi-keystone-api-version-tests

Change-Id: I8c38fb8d191a88fab37633c6edffd54492c41fd5
diff --git a/tempest/common/rest_client.py b/tempest/common/rest_client.py
index 02a8c65..10223a0 100644
--- a/tempest/common/rest_client.py
+++ b/tempest/common/rest_client.py
@@ -143,6 +143,10 @@
         return self.auth_provider.credentials.username
 
     @property
+    def user_id(self):
+        return self.auth_provider.credentials.user_id
+
+    @property
     def tenant_name(self):
         return self.auth_provider.credentials.tenant_name