Returns list of instances unreachable via ssh.

def GetUnreachableInstances(instances, ssh_key):
  """ Returns list of instances unreachable via ssh. """
  hostnames = [i.private_ip for i in instances]
  ssh_status = AreHostsReachable(hostnames, ssh_key)
  assert(len(hostnames) == len(ssh_status))
  nonresponsive_instances = [instance for (instance, ssh_ok) in
                             zip(instances, ssh_status) if not ssh_ok]
  return nonresponsive_instances