Bug for heat template: ResourceGroup中的refs属性不支持索引

https://bugs.launchpad.net/heat/+bug/1434586

minimal reproducer:

$ cat refs.yaml
heat_template_version: 2015-04-30
resources:
  group:
    type: OS::Heat::ResourceGroup
    properties:
      count: 2
      resource_def:
        type: OS::Heat::RandomString
outputs:
  allrefs:
    value: { get_attr: [ group, refs ] }
  oneref:
    value: { get_attr: [group, refs, 0 ] }

$ heat stack-create refs -f refs.yaml
$ heat stack-list
+--------------------------------------+------------+-----------------+----------------------+
| id | stack_name | stack_status | creation_time |
+--------------------------------------+------------+-----------------+----------------------+
| 5a7dec43-ebe8-46e5-a1e0-46871bd67da4 | refs | CREATE_COMPLETE | 2015-03-20T14:07:31Z |
+--------------------------------------+------------+-----------------+----------------------+

$ heat output-show refs allrefs
[
  "a4t2y5IuYLJzEZRVdFCEKhAnaWfI9vTi",
  "5HwxHxZN41J78l42OKH07igJif8Pk7tn"
]

$ heat output-show refs oneref
[
  "a4t2y5IuYLJzEZRVdFCEKhAnaWfI9vTi",
  "5HwxHxZN41J78l42OKH07igJif8Pk7tn"
]