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/api/compute/v3/admin/test_quotas.py b/tempest/api/compute/v3/admin/test_quotas.py
index 27836fb..19c31fe 100644
--- a/tempest/api/compute/v3/admin/test_quotas.py
+++ b/tempest/api/compute/v3/admin/test_quotas.py
@@ -32,7 +32,7 @@
 
         # NOTE(afazekas): these test cases should always create and use a new
         # tenant most of them should be skipped if we can't do that
-        cls.demo_tenant_id = cls.isolated_creds.get_primary_creds().tenant_id
+        cls.demo_tenant_id = cls.client.tenant_id
 
         cls.default_quota_set = set(('metadata_items',
                                      'ram', 'floating_ips',