Heat Native Resource Group

Based on a conversation between myself and Chris Armstrong. Basically this resource allows for creating groups of identically configured, arbitrary resource. Proposed template syntax is:

resources:
  group1:
    type: OS::Heat::ResourceGroup
    properties:
      count: 2
      resource_def:
          type: OS::Nova::Server # or any other known resource type
          properties:
            image: F19-cfntools
            flavor: x-large
            user_data: (...)

This resource will then construct a nested template and copypasta the resource_def into the nested template "count" times. This resource would mirror the attributes of the nested resource and present an agrigate attribute value for each attribute of the nested resource. It would also allow for retrieving an attribute of a specific nested resource via synthetic attributes of the form "resource.<index>.<attribute name>.

Gerrit topic: https://review.openstack.org/#q,topic:bp/native-resource-group,n,z

Addressed by: https://review.openstack.org/51759
    Implement a Heat-native resource group