add _listeners_from_job method to Conductor base

Change-Id: I4a58c2fd5a0c648ffe3f81e2170a622b6b0b43f0

taskflow/conductors/base.py

@@ -88,10 +88,28 @@ class Conductor(object):
             store = dict(job.details["store"])
         else:
             store = {}
-        return engines.load_from_detail(flow_detail, store=store,
-                                        engine=self._engine,
-                                        backend=self._persistence,
-                                        **self._engine_options)
+        engine = engines.load_from_detail(flow_detail, store=store,
+                                          engine=self._engine,
+                                          backend=self._persistence,
+                                          **self._engine_options)
+        for listener in self._listeners_from_job(job, engine):
+            listener.register()
+        return engine
+
+    def _listeners_from_job(self, job, engine):
+        """Returns a list of listeners to be attached to an engine.
+
+        This method should be overridden in order to attach listeners to
+        engines. It will be called once for each job, and the list returned
+        listeners will be added to the engine for this job.
+
+        :param job: A job instance that is about to be run in an engine.
+        :param engine: The engine that listeners will be attached to.
+        :returns: a list of (unregistered) listener instances.
+        """
+        # TODO(dkrause): Create a standard way to pass listeners or
+        #                listener factories over the jobboard
+        return []
 
     @lock_utils.locked
     def connect(self):