Add strategy_id & goal_id fields in audit template

In this changeset, I updated the 'goal_id' field into the AuditTemplate
to now become a mandatory foreign key towards the Goal model. I also
added the 'strategy_id' field into the AuditTemplate model to be an
optional foreign key onto the Strategy model.

This changeset also includes an update of the /audit_template
Watcher API endpoint to reflect the previous changes.

As this changeset changes the API, this should be merged alongside the
related changeset from python-watcherclient.

Partially Implements: blueprint get-goal-from-strategy

Change-Id: Ic0573d036d1bbd7820f8eb963e47912d6b3ed1a9

watcher/decision_engine/strategy/context/default.py

@@ -52,8 +52,8 @@ class DefaultStrategyContext(base.BaseStrategyContext):
         cluster_data_model = collector_manager.get_latest_cluster_data_model()
 
         strategy_selector = default.DefaultStrategySelector(
-            goal_name=objects.Goal.get_by_name(
-                request_context, audit_template.goal).name,
+            goal_name=objects.Goal.get_by_id(
+                request_context, audit_template.goal_id).name,
             strategy_name=None,
             osc=osc)