Bug 1332506 - Treeherder actions defined in-tree. r=dustin

Framework for defining actions in-tree that can be displayed
and triggered from Treeherder.

MozReview-Commit-ID: 3rvwgy2i4xu

taskcluster/mach_commands.py

@@ -302,6 +302,12 @@ class MachCommands(MachCommandBase):
             traceback.print_exc()
             sys.exit(1)
 
+    @SubCommand('taskgraph', 'action-callback',
+                description='Run action callback used by action tasks')
+    def action_callback(self, **options):
+        import actions
+        actions.trigger_action_callback()
+
     def setup_logging(self, quiet=False, verbose=True):
         """
         Set up Python logging for all loggers, sending results to stderr (so