Launch a GCE instance.

def launch_instance(instance_name,
                    command,
                    existing_ip=None,
                    cpu=1,
                    mem=4,
                    code_dir=None,
                    setup_command=None):
  """Launch a GCE instance."""
  # Create instance
  ip = existing_ip or create_instance(instance_name, cpu=cpu, mem=mem)
  tf.logging.info("Waiting for SSH %s", instance_name)
  ready = wait_for_ssh(ip)
  if not ready:
    raise ValueError("Instance %s never ready for SSH" % instance_name)

  # Copy code
  if code_dir:
    shell_run_with_retry(COPY_CODE, retries=2,
                         local_dir=code_dir, instance_name=instance_name)

  # Run setup
  if setup_command:
    tf.logging.info("Running setup on %s", instance_name)
    remote_run(setup_command, instance_name)

  # Run command
  tf.logging.info("Running command on %s", instance_name)
  remote_run(command, instance_name, detach=True)