provider/gce: respect region for zones.
provider/gce: Implement environ.StopInstances.
provider/gce: Finish the environProvider.Prepare implementation.
provider/gce: Fix the machineType for newInstance call.
provider/gce: Clarify simplestreams endpoint stuff.
provider/gce: machineId -> name.
provider/gce: Move common port-related method implementations onto environ.
provider/gce: Implement the port-related environ methods.