Use constants for revert automatically provided kwargs

Instead of using strings use module level constants for
the automatically provided keyword arguments to the tasks
revert function. This makes it easier for users of taskflow
to associate these constants with the actual keywords, without
having to resort to using raw strings directly.

Change-Id: I12726812615052d8405071d46272ef2b8286cfe2

taskflow/engines/action_engine/executor.py

@@ -19,6 +19,7 @@ import abc
 from concurrent import futures
 import six
 
+from taskflow import task as _task
 from taskflow.utils import async_utils
 from taskflow.utils import misc
 from taskflow.utils import threading_utils
@@ -44,8 +45,8 @@ def _execute_task(task, arguments, progress_callback):
 
 def _revert_task(task, arguments, result, failures, progress_callback):
     kwargs = arguments.copy()
-    kwargs['result'] = result
-    kwargs['flow_failures'] = failures
+    kwargs[_task.REVERT_RESULT] = result
+    kwargs[_task.REVERT_FLOW_FAILURES] = failures
     with task.autobind('update_progress', progress_callback):
         try:
             task.pre_revert()

taskflow/task.py

@@ -27,6 +27,14 @@ from taskflow.utils import reflection
 
 LOG = logging.getLogger(__name__)
 
+# Constants passed into revert kwargs.
+#
+# Contain the execute() result (if any).
+REVERT_RESULT = 'result'
+#
+# The cause of the flow failure/s
+REVERT_FLOW_FAILURES = 'flow_failures'
+
 
 @six.add_metaclass(abc.ABCMeta)
 class BaseTask(atom.Atom):