docs: def all_nodes_that_run_in_env(service, env, service_configuration=None):
      """ Returns all nodes that run in an environment. This needs
      to be specified in field named 'env_runs_on' one level under services
      in the configuration, and needs to contain an object which maps strings
      to lists (environments to nodes).
  
      :param service: A string specifying which service to look up nodes for
      :param env: A string specifying which environment's nodes should be returned
      :param service_configuration: A service_configuration dict to look in or None to
                                    use the default dict.
  
      :returns: list of all nodes running in a certain environment
      """
  
      if service_configuration is None:
          service_configuration = read_services_configuration()
      env_runs_on = service_configuration[service]['env_runs_on']
      if env in env_runs_on:
          return list(sorted(env_runs_on[env]))
      else:
          return []