Adapt Watcher to Python3.7

There are some issues with bundle of eventlet, taskflow and
apscheduler on Python 3.7.
According to [0], replace ThreadPoolExecutor with
GreenThreadPoolExecutor

[0]: http://lists.openstack.org/pipermail/openstack-dev/2018-July/132481.html

Co-Authored-By: Canwei Li <li.canwei2@zte.com.cn>

Change-Id: I989d7155eb8764088f91b3ca0d1f47ac6332bf11

watcher/applier/workflow_engine/default.py

@@ -106,7 +106,7 @@ class DefaultWorkFlowEngine(base.BaseWorkFlowEngine):
                               decider=self.decider)
 
             e = engines.load(
-                flow, engine='parallel',
+                flow, executor='greenthreaded', engine='parallel',
                 max_workers=self.config.max_workers)
             e.run()