Delete user creds from DB when no longer required

The user credentials are only for a particular stack and should be deleted
when that stack is deleted.

Fixes #162.

Change-Id: Id5d1f101df17ede1843e4f2fca51903c956667ca
Signed-off-by: Zane Bitter <zbitter@redhat.com>

heat/db/sqlalchemy/api.py

@@ -166,9 +166,11 @@ def stack_delete(context, stack_id):
         session.delete(r)
 
     rt = s.raw_template
+    uc = s.user_creds
 
     session.delete(s)
     session.delete(rt)
+    session.delete(uc)
 
     session.flush()
 

heat/db/sqlalchemy/models.py

@@ -172,6 +172,8 @@ class UserCreds(BASE, HeatBase):
     aws_auth_url = Column(String)
     tenant_id = Column(String)
     aws_creds = Column(String)
+    stack = relationship(Stack,
+        backref=backref('user_creds'))
 
 
 class Event(BASE, HeatBase):