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>