Dump a yaml file containing client info.

def DumpClientYaml(client_urn, target_dir, token=None, overwrite=False):
  """Dump a yaml file containing client info."""
  fd = aff4.FACTORY.Open(client_urn, aff4_grr.VFSGRRClient, token=token)
  dirpath = os.path.join(target_dir, fd.urn.Split()[0])
  try:
    # Due to threading this can actually be created by another thread.
    os.makedirs(dirpath)
  except OSError:
    pass
  filepath = os.path.join(dirpath, "client_info.yaml")
  if not os.path.isfile(filepath) or overwrite:
    with open(filepath, "wb") as out_file:
      out_file.write(serialize.YamlDumper(fd))