Avoid forcing engine_conf to a dict

It is valid to provide engine_conf that is None and
when a engine is loaded it will notice that it is None
and load its default configuration.

Change-Id: Icde7ac85921f3b434cee39a9dfe7de997f2120bf

taskflow/conductors/base.py

@@ -81,10 +81,9 @@ class Conductor(object):
             store = dict(job.details["store"])
         else:
             store = {}
-        engine_conf = dict(self._engine_conf)
         return taskflow.engines.load_from_detail(flow_detail,
                                                  store=store,
-                                                 engine_conf=engine_conf,
+                                                 engine_conf=self._engine_conf,
                                                  backend=self._persistence)
 
     @lock_utils.locked