Access credential fields as attributes

Some tests access credentials attributes using dictionary
syntax (e.g. with get method); changed to use the Credentials
attribute directly.

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

Change-Id: Ic5b68af18d9a2dda581bdc1500d064e0ba732ec4
diff --git a/tempest/api/volume/admin/test_volume_quotas.py b/tempest/api/volume/admin/test_volume_quotas.py
index 2949d56..531e145 100644
--- a/tempest/api/volume/admin/test_volume_quotas.py
+++ b/tempest/api/volume/admin/test_volume_quotas.py
@@ -29,8 +29,7 @@
     def setUpClass(cls):
         super(VolumeQuotasAdminTestJSON, cls).setUpClass()
         cls.admin_volume_client = cls.os_adm.volumes_client
-        cls.demo_tenant_id = cls.isolated_creds.get_primary_user().get(
-            'tenantId')
+        cls.demo_tenant_id = cls.isolated_creds.get_primary_creds().tenant_id
 
     @test.attr(type='gate')
     def test_list_quotas(self):