Bug 1465970 - make backfill action a hook; r=tomprince

MozReview-Commit-ID: 1ibZPb5fMdP

--HG--
extra : rebase_source : 1a3deae3f1deacc4e06d6e4bb3fb95269295821e

taskcluster/taskgraph/actions/backfill.py

@@ -25,6 +25,8 @@ logger = logging.getLogger(__name__)
 @register_callback_action(
     title='Backfill',
     name='backfill',
+    kind='hook',
+    generic=True,
     symbol='Bk',
     description=('Take the label of the current task, '
                  'and trigger the task with that label '