Use show_volume_type to check the result of update_volume_type

In test_volume_type_update, after calling update_volume_type, it
only checks the response body of update_volume_type, and we should
call show_volume_type to check whether the volume type is really
updated.

Change-Id: I6b7a58a47bab22286e4ff783e9053def4f692ecc
diff --git a/tempest/api/volume/admin/test_volume_types.py b/tempest/api/volume/admin/test_volume_types.py
index 1077524..9e24176 100644
--- a/tempest/api/volume/admin/test_volume_types.py
+++ b/tempest/api/volume/admin/test_volume_types.py
@@ -193,8 +193,13 @@
                   'is_public': is_public}
         updated_vol_type = self.admin_volume_types_client.update_volume_type(
             volume_type['id'], **kwargs)['volume_type']
-
-        # Verify volume type details were updated
         self.assertEqual(name, updated_vol_type['name'])
         self.assertEqual(description, updated_vol_type['description'])
         self.assertEqual(is_public, updated_vol_type['is_public'])
+
+        # Verify volume type details were updated
+        fetched_volume_type = self.admin_volume_types_client.show_volume_type(
+            volume_type['id'])['volume_type']
+        self.assertEqual(name, fetched_volume_type['name'])
+        self.assertEqual(description, fetched_volume_type['description'])
+        self.assertEqual(is_public, fetched_volume_type['is_public'])