Add started_at and finished_at to task execution.

Sometimes it is very important to know exact time of task execution, but using fields
created_at and updated_at is incorrect, because in this case duration will consist
of not only Running time.
That new fields solve this problem.

Change-Id: I15be0648a0346f5b3dc9ef4a1b330a6c0e818385
Implements: blueprint mistral-add-started-finished-at
Signed-off-by: Oleg Ovcharuk <vgvoleg@gmail.com>

mistral/workflow/base.py

@@ -144,7 +144,8 @@ class WorkflowController(object):
             return []
 
         cmds = [
-            commands.RunExistingTask(self.wf_ex, self.wf_spec, t_e, reset)
+            commands.RunExistingTask(self.wf_ex, self.wf_spec, t_e, reset,
+                                     rerun=True)
             for t_e in task_execs
         ]