Added action_plan ObjectField for Action

In this changeset, I added the "action_plan" ObjectField which can
either be loaded by setting the new "eager" parameter as True
or not loaded (as before) by setting it to False. The advantage of
introducing this eager parameter is that this way,
we can reduce to a minimum the overhead of DB queries whenever the
related object fields is not actually needed.

Change-Id: Iae255d9a0a919e2b6db710be24dbf9033b72166e
Partially-Implements: blueprint watcher-versioned-objects

watcher/tests/applier/action_plan/test_default_action_handler.py

@@ -26,6 +26,7 @@ from watcher.tests.objects import utils as obj_utils
 
 
 class TestDefaultActionPlanHandler(base.DbTestCase):
+
     def setUp(self):
         super(TestDefaultActionPlanHandler, self).setUp()
         obj_utils.create_test_goal(self.context)