Refactoring workflow handler

* Introduced new class Workflow that manages life-cycle of running
  workflows and is responsible for managing workflow persistent state
* Moved all workflow level logic to workflow handler and Workflow class
* Changed semantics if how workflows start errors are handled.
  Previously, in case of invalid user input Mistral engine would store
  information about error in "state_info" field of workflow execution
  and bubble up an exception to the user. This approach was incorrect
  for a number of reasons including broken semantics: if an exception
  was raised due to invalid input it's normal to expect that system
  state has not changed. After this refactoring, engine only raises
  an exception in case of bad user input. That way behavior is
  consistent with the idea of exceptional situations.
* Fixed unit tests in according to the previous point
* Fixed a number of logical issues in tests. For example, in
  test_default_engine.py we expected one type of errors (e.g. env not
  found) but effectively received another one (invalid input).

Partially implements: blueprint mistral-engine-error-handling

Change-Id: I09070411fd833df8284cb80db69b8401a40eb6fe

mistral/workflow/base.py

@@ -111,6 +111,8 @@ class WorkflowController(object):
         if self._is_paused_or_completed():
             return []
 
+        # TODO(rakhmerov): I think it should rather be a new method
+        # rerun_task() because it covers a different use case.
         if task_ex:
             return self._get_rerun_commands([task_ex], reset, env=env)
 
@@ -199,7 +201,11 @@ class WorkflowController(object):
         :param env: A set of environment variables to overwrite.
         :return: List of workflow commands.
         """
+
         for task_ex in task_exs:
+            # TODO(rakhmerov): It is wrong that we update something in
+            # workflow controller, by design it should not change system
+            # state. Fix it, it should happen outside.
             self._update_task_ex_env(task_ex, env)
 
         cmds = [commands.RunExistingTask(t_e, reset) for t_e in task_exs]