Add ability to combine ID maps for multiple resource groups

[Jay Dobies]

This came up as a request on https://bugs.launchpad.net/heat/+bug/1582837. I'm splitting this out into its own feature. The relevant request is below:

Can we please figure out a way (e.g via map_merge or repeat) to combine the maps of ID's from multiple resource groups, such that it's possible to do this:

group1:
    type: OS::Heat::ResourceGroup
    properties:
      count: {get_param: count_group1}
      resource_def:
        type: OS::Nova::Server
        properties:
          key_name: { get_param: key_name }
          image: { get_param: image }
          flavor: { get_param: flavor }

group2:
    type: OS::Heat::ResourceGroup
    properties:
      count: {get_param: count_group2}
      resource_def:
        type: OS::Nova::Server
        properties:
          key_name: { get_param: key_name }
          image: { get_param: image }
          flavor: { get_param: flavor }

allnodes_deployment_group:
    type: OS::Heat::SoftwareDeploymentGroup
    properties:
      servers:
        <map_merge or repeat?>
        - { get_attr: [group1, $resource_id_map_attribute] }
        - { get_attr: [group2, $resource_id_map_attribute] }

Note we can't just use map_merge, because the keys will collide, so perhaps we can make repeat capable of generating maps, and iterating over each map adding a prefix related to the group name?