Splitting executions into different tables

* Having different types of execution objects in different
  tables will give less contention on DB tables and hence better
  performance so DB schema was changed accordingly
* Fixed all unit tests and places in the code where we assumed
  polymorphic access to execution objects
* Other minor fixes

TODO(in upcoming patches):
* DB migration script

Change-Id: Ibc8408e12dd85e143302d7fdddace32954551ac5

mistral/tests/unit/workflow/test_direct_workflow.py

@@ -96,7 +96,7 @@ class DirectWorkflowControllerTest(base.DbTestCase):
 
         get_task_execution.return_value = task1_ex
 
-        task1_ex.executions.append(
+        task1_ex.action_executions.append(
             models.ActionExecution(
                 name='std.echo',
                 workflow_name='wf',
@@ -119,7 +119,7 @@ class DirectWorkflowControllerTest(base.DbTestCase):
 
         # Now assume that 'task2' completed successfully.
         task2_ex = self._create_task_execution('task2', states.SUCCESS)
-        task2_ex.executions.append(
+        task2_ex.action_executions.append(
             models.ActionExecution(
                 name='std.echo',
                 workflow_name='wf',

mistral/tests/unit/workflow/test_with_items.py

@@ -31,17 +31,21 @@ class WithItemsTest(base.BaseTest):
     def test_get_indices(self):
         # Task execution for running 6 items with concurrency=3.
         task_ex = models.TaskExecution(
+            spec={
+                'action': 'myaction'
+            },
             runtime_context={
                 'with_items_context': {
                     'capacity': 3,
                     'count': 6
                 }
             },
-            executions=[]
+            action_executions=[],
+            workflow_executions=[]
         )
 
         # Set 3 items: 2 success and 1 error unaccepted.
-        task_ex.executions += [
+        task_ex.action_executions += [
             self.get_action_ex(True, states.SUCCESS, 0),
             self.get_action_ex(True, states.SUCCESS, 1),
             self.get_action_ex(False, states.ERROR, 2)