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/common/exception.py

@@ -154,6 +154,10 @@ class InvalidGoal(Invalid):
     msg_fmt = _("Goal %(goal)s is invalid")
 
 
+class InvalidStrategy(Invalid):
+    msg_fmt = _("Strategy %(strategy)s is invalid")
+
+
 class InvalidUUID(Invalid):
     msg_fmt = _("Expected a uuid but received %(uuid)s")