Add workflow_execution_id to task notification object

Change-Id: I76ab5526bd4444059f5163b3767cfebcc37b7a01
Signed-off-by: Oleg Ovcharuk <vgvoleg@gmail.com>

mistral/engine/tasks.py

@@ -80,6 +80,7 @@ class Task(object):
             return {
                 "id": self.task_ex.id,
                 "name": self.task_ex.name,
+                "workflow_execution_id": self.task_ex.workflow_execution_id,
                 "workflow_name": self.task_ex.workflow_name,
                 "workflow_namespace": self.task_ex.workflow_namespace,
                 "workflow_id": self.task_ex.workflow_id,