Run command on GCS instance, optionally detached.

def remote_run(cmd, instance_name, detach=False, retries=1):
  """Run command on GCS instance, optionally detached."""
  if detach:
    cmd = SCREEN.format(command=cmd)
  args = SSH.format(instance_name=instance_name).split()
  args.append(cmd)
  for i in range(retries + 1):
    try:
      if i > 0:
        tf.logging.info("Retry %d for %s", i, args)
      return sp.check_call(args)
    except sp.CalledProcessError as e:
      if i == retries:
        raise e