Add nested resource references to resource group

Originally, there was no way to get references to the
resources inside the resource group. This prevented
things like attaching volumes to instances in the group.
This patch allows both aggregated as well as indexed
access to ref id's of the nested resources.

Closes-Bug: #1259830
Change-Id: Id13eb6edd91bfc34d8bab05f429e761fe89ed799
Merge "Add nested resource references to resource group"