Code review comment for lp:~jimbaker/pyjuju/expose-provider-ec2

> Changed such that ProviderMachine now takes an optional machine_id so this
> can be passed around. In general, this field needs to be assigned after the
> fact (EC2 doesn't know these machine IDs from our topology), and this is
> what is done in the provisioning agent. But it does provide a central place
> for this information instead of having to pass around two related pieces.

Cool. One day, it would be good to ensure we do always have machine_id set on ProviderMachine instances, but no clean way to do so springs to mind now, so I don't think it blocks anything.

Everything else looks sensible to me, +1. (And I definitely like the operations as functions :))