Use LOG.exception() properly

Contrary to popular belief, LOG.exception() is not a method to which you
pass an exception in order to log it. Rather, you pass the message to be
logged at ERROR level, and the exception is retrieved automatically via
sys.exc_info().

Change-Id: I197cf94ada34a7ce80fc4026a99d95cd50823882
diff --git a/scenario/test_volumes.py b/scenario/test_volumes.py
index 7980d81..603c8f2 100644
--- a/scenario/test_volumes.py
+++ b/scenario/test_volumes.py
@@ -77,9 +77,8 @@
                 template_name='test_volumes_create_from_backup.yaml',
                 add_parameters={'backup_id': backup.id})
             stack2 = self.client.stacks.get(stack_identifier2)
-        except exceptions.StackBuildErrorException as e:
-            LOG.error("Halting test due to bug: #1382300")
-            LOG.exception(e)
+        except exceptions.StackBuildErrorException:
+            LOG.exception("Halting test due to bug: #1382300")
             return
 
         # Verify with cinder that the volume exists, with matching details