Unskip test_stack_update_add_remove()

test_stack_update_add_remove was skipped because of bug 1308682 which
is now in a state which should allow us to unskip it.

Change-Id: I9b728abe3432f61f883d50b4a9d7d8d9c2b1f8b2
Related-Bug: #1308682
diff --git a/tempest/api/orchestration/stacks/test_update.py b/tempest/api/orchestration/stacks/test_update.py
index 791a19b..98761ac 100644
--- a/tempest/api/orchestration/stacks/test_update.py
+++ b/tempest/api/orchestration/stacks/test_update.py
@@ -61,7 +61,6 @@
                          self.list_resources(stack_identifier))
 
     @test.attr(type='gate')
-    @test.skip_because(bug='1308682')
     def test_stack_update_add_remove(self):
         stack_name = data_utils.rand_name('heat')
         stack_identifier = self.create_stack(stack_name, self.template)