Fix pylint warning for format string

Change-Id: Ic53997bc257aa690a5c00b19c3cbc7e8d973959f
diff --git a/drivers/Dummy.py b/drivers/Dummy.py
index 41f36af..7750dd5 100644
--- a/drivers/Dummy.py
+++ b/drivers/Dummy.py
@@ -113,7 +113,7 @@
             del self._dummy_vlans[tag]
         else:
             # It's not an error if it doesn't exist, but log anyway
-            logging.debug("VLAN %d did not exist" % tag)
+            logging.debug("VLAN %d did not exist", tag)
 
     # Set the name of a VLAN
     def vlan_set_name(self, tag, name):