Added 'goal' ObjectField for Strategy object

In this changeset, I added the "goal" 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.

Partially-Implements: blueprint watcher-versioned-objects
Change-Id: I103c9ed161d2cedf7b43c55f9e095ef66bf44dea

watcher/tests/db/utils.py

@@ -199,7 +199,7 @@ def create_test_scoring_engine(**kwargs):
 
 
 def get_test_strategy(**kwargs):
-    return {
+    strategy_data = {
         'id': kwargs.get('id', 1),
         'uuid': kwargs.get('uuid', 'cb3d0b58-4415-4d90-b75b-1e96878730e3'),
         'name': kwargs.get('name', 'TEST'),
@@ -208,9 +208,16 @@ def get_test_strategy(**kwargs):
         'created_at': kwargs.get('created_at'),
         'updated_at': kwargs.get('updated_at'),
         'deleted_at': kwargs.get('deleted_at'),
-        'parameters_spec': kwargs.get('parameters_spec', {})
+        'parameters_spec': kwargs.get('parameters_spec', {}),
     }
 
+    # goal ObjectField doesn't allow None nor dict, so if we want to simulate a
+    # non-eager object loading, the field should not be referenced at all.
+    if kwargs.get('goal'):
+        strategy_data['goal'] = kwargs.get('goal')
+
+    return strategy_data
+
 
 def get_test_service(**kwargs):
     return {