Change LOG.warn to LOG.warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning
so we prefer to use warning to avoid DeprecationWarning.

Change-Id: I2c7db9f6a97b131700c3aad5d49b6a206141f34b
Closes-Bug: #1530742

mistral/engine/default_engine.py

@@ -380,7 +380,7 @@ class DefaultEngine(base.Engine, coordination.Service):
             try:
                 final_context = wf_ctrl.evaluate_workflow_final_context()
             except Exception as e:
-                LOG.warn(
+                LOG.warning(
                     "Failed to get final context for %s: %s" % (wf_ex, e)
                 )
             return wf_handler.succeed_workflow(

mistral/engine/default_executor.py

@@ -76,7 +76,7 @@ class DefaultExecutor(base.Executor, coordination.Service):
                    " Actual init params = %s. More info: %s"
                    % (action_class_str, i_u.get_arg_list(action_cls.__init__),
                       action_params.keys(), e))
-            LOG.warn(msg)
+            LOG.warning(msg)
 
         except exc.ActionException as e:
             msg = ("Failed to run action [action_ex_id=%s, action_cls='%s',"