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/db/sqlalchemy/models.py

@@ -152,7 +152,8 @@ class AuditTemplate(Base):
     name = Column(String(63), nullable=True)
     description = Column(String(255), nullable=True)
     host_aggregate = Column(Integer, nullable=True)
-    goal = Column(String(63), nullable=True)
+    goal_id = Column(Integer, ForeignKey('goals.id'), nullable=False)
+    strategy_id = Column(Integer, ForeignKey('strategies.id'), nullable=True)
     extra = Column(JSONEncodedDict)
     version = Column(String(15), nullable=True)