StopInstances previously took []instance.Instance, which is unnecessary for most providers, and imposes an unnecessary cost for callers that only have IDs initially. We change StopInstances to
The openstack provider's StopInstances is now slightly more expensive than before, requiring an additional call to Instances. This cost will disappear when we fix the security groups/open-port implementation.
Drive-by: the maas provider now uses a bulk "release" in StopInstances.
Live tested on ec2, local, virtual MAAS, canonistack, azure. I lack a Joyent account, but I think it should be fine.