hook for destruction

[Clint Byrum]

Excerpts from Marcos Barbosa's message of 2012-11-14 07:12:11 -0800:
> There's a little theory about RDS. A DB instance is  like a EC2 with MySQL.
> Inside DB instance we can create databases. I already have information
> about when the charm is deployed (install hook) to create DB instance, but
> I need know when the environment is destroyed  to delete DB instance.
> 

I think this succinctly spells out a reason that there is a need for
something beyond subordinates to define "virtual" or maybe "external"
services such as RDS or ELB. I opened a bug about this a long time ago:

https://bugs.launchpad.net/juju/+bug/878948

For now, you must keep track of your virtual services and manually
destroy them.

I also wonder if we should recommend that subordinates which implement
access to these for-pay type services not allocate the resources, so
its clear that the allocation and de-allocation happen outside of charms.