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

* Tiny, irrelevant note: rather than directly creating an EC2ProviderMachine, there's now a machine_from_instance function in ec2.machine which is a touch more convenient.

* Significant issue: I don't like having both 'machine' and 'machine_id' parameters: if there isn't currently a way to determine machine_id given a machine, I think it would be worth adding one and using it, to avoid cluttering up the Provider interface.