Updates the password or privilege-level for a user.

def UpdateUser(username, password=None, is_admin=False):
  """Updates the password or privilege-level for a user."""
  user_type, password = _GetUserTypeAndPassword(
      username, password=password, is_admin=is_admin)
  grr_api = maintenance_utils.InitGRRRootAPI()
  grr_user = grr_api.GrrUser(username).Get()
  grr_user.Modify(user_type=user_type, password=password)