heat engine : DBInstance fix so nested stack is deleted

Currently the RDS DBInstance resource inherits from stack.Stack but it
should be stack.NestedStack, which implements handle_delete whereas
stack.Stack does not, meaning the DBInstance nested stack is never
deleted when the owning stack gets deleted.

fixes bug 1097299

Change-Id: I865611d4de9ea4c3caa43c7a11a28924602a5b0d
Signed-off-by: Steven Hardy <shardy@redhat.com>

heat/engine/resources/dbinstance.py

@@ -163,7 +163,7 @@
 '''
 
 
-class DBInstance(stack.Stack):
+class DBInstance(stack.NestedStack):
 
     properties_schema = {
         'DBSnapshotIdentifier': {'Type': 'String',