modifies the test to sort based on UUID

This patch modifies the volume-list filter test to perform the sorting
based on UUID instead of created_at timestamp as this timestamp might
be same for two volumes but UUID for every volume is unique.

Change-Id: Ibe1ab23c12ad8afe8d5adab6fe0c617445adc90d
Closes-Bug: 1313970
diff --git a/tempest/api/volume/v2/test_volumes_list.py b/tempest/api/volume/v2/test_volumes_list.py
index 41445d7..e90c957 100644
--- a/tempest/api/volume/v2/test_volumes_list.py
+++ b/tempest/api/volume/v2/test_volumes_list.py
@@ -203,7 +203,7 @@
         def _list_details_with_multiple_params(limit=2,
                                                status='available',
                                                sort_dir='asc',
-                                               sort_key='created_at'):
+                                               sort_key='id'):
             params = {'limit': limit,
                       'status': status,
                       'sort_dir': sort_dir,