Update a config with a random csrf key.

def _GenerateCSRFKey(config):
  """Update a config with a random csrf key."""
  secret_key = config.Get("AdminUI.csrf_secret_key", None)
  if not secret_key:
    # TODO(amoser): Remove support for django_secret_key.
    secret_key = config.Get("AdminUI.django_secret_key", None)
    if secret_key:
      config.Set("AdminUI.csrf_secret_key", secret_key)

  if not secret_key:
    key = utils.GeneratePassphrase(length=100)
    config.Set("AdminUI.csrf_secret_key", key)
  else:
    print("Not updating csrf key as it is already set.")