Add lists of instance objects

This adds a list-of-instances object, which can house the "get all"
methods for instances. It's very simple, emulates a list, but
provides us an object-oriented way to get multiple instances that
mirrors the model.

Related to blueprint unified-object-model

Change-Id: Id6579030b88b92d4cb7db13ee6718ad01db7bf63
Merge "Add lists of instance objects"