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/messaging/trigger.py

@@ -16,7 +16,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-from concurrent import futures
+import futurist
 
 from oslo_config import cfg
 from oslo_log import log
@@ -31,7 +31,7 @@ class TriggerActionPlan(object):
     def __init__(self, applier_manager):
         self.applier_manager = applier_manager
         workers = CONF.watcher_applier.workers
-        self.executor = futures.ThreadPoolExecutor(max_workers=workers)
+        self.executor = futurist.GreenThreadPoolExecutor(max_workers=workers)
 
     def do_launch_action_plan(self, context, action_plan_uuid):
         try:

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()