Added goal+strategy ObjectField for AuditTemplate

In this changeset, I added the "goal" and the "strategy "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 goal is not actually needed.

Change-Id: I97418163f68aea2f1cda80be94e4035f0b3700ae
Partially-Implements: blueprint watcher-versioned-objects

watcher/tests/decision_engine/messaging/test_audit_endpoint.py

@@ -27,6 +27,7 @@ from watcher.tests.objects import utils as obj_utils
 class TestAuditEndpoint(base.DbTestCase):
     def setUp(self):
         super(TestAuditEndpoint, self).setUp()
+        self.goal = obj_utils.create_test_goal(self.context)
         self.audit_template = obj_utils.create_test_audit_template(
             self.context)
         self.audit = obj_utils.create_test_audit(