Fixed formatting error that caused migration to fail.

Author: Simon Van den Bossche

ovs/update/volumedriver/updater.py

@@ -266,7 +266,7 @@ def migrate_master_mds(cls, storagerouter, max_chain_size=100, group_timeout=10
                         task.revoke()
                         revoked_tasks.append(task)
                 if revoked_tasks:
-                    cls.logger.warning('Revoked migration tasks: {}'.format(', '.join(revoked_tasks)))
+                    cls.logger.warning('Revoked migration tasks: {}'.format(', '.join(t.id for t in revoked_tasks)))
                 cls.logger.warning('Waiting for the execution on the running migrations')
                 _ = async_result.get()
         cls.logger.info("MDS migration finished")