isolate representation from content in GET request

Container server representation of data (received from backend) is isolated
into `create_listing()` call. GET() method will call `create_listing()`
when it's ready to create a serialized object listing. It will allow us to
customize that representation: get different listing, if backend supports
more interesting queries, for example.

Change-Id: I4fa03357d3d9e697bc3e1fb5da42d7572c6ae9b3
Merge "isolate representation from content in GET request"