Unstable test_container_synchronization

The bug which was the reason for skipping the test is considered
resolved. To make sure the test works, let's mark it unstable
for a while.

Change-Id: I7f02d82f5758bc2e32b06234652f755512a74787
Related-Bug: #1317133
diff --git a/tempest/api/object_storage/test_container_sync.py b/tempest/api/object_storage/test_container_sync.py
index 322579c..bdcb5ae 100644
--- a/tempest/api/object_storage/test_container_sync.py
+++ b/tempest/api/object_storage/test_container_sync.py
@@ -123,7 +123,7 @@
                 self.assertEqual(object_content, obj_name[::-1].encode())
 
     @decorators.attr(type='slow')
-    @decorators.skip_because(bug='1317133')
+    @decorators.unstable_test(bug='1317133')
     @decorators.idempotent_id('be008325-1bba-4925-b7dd-93b58f22ce9b')
     @testtools.skipIf(
         not CONF.object_storage_feature_enabled.container_sync,