Comment 6 for bug 1344284

I'm having a similar issue with joining attributes of ResourceGroup's inner resources:

  servers:
    type: OS::Heat::ResourceGroup
    properties:
      count: 3
      resource_def:
        type: OS::Nova::Server
        properties:
          image: overcloud-compute
          flavor: baremetal

  allNodesConfig:
    type: OS::Heat::StructuredConfig
    properties:
      config:
        completion-signal:
          get_input: deploy_signal_id
        names:
          list_join:
          - ','
          - {get_attr: [servers, name]}

heat stack-create reports: ERROR: Property error : allNodesConfig: config "list_join" must operate on a list.