objects: allow creation of objects without dict item compat

The dict compat support in NovaObject is useful for incrementally
converting existing code over to use objects. Any brand new objects
though have no reason to support dictionary access, so there should
be a way to disable this compat mode.

This moves all the dict compat support methods out of NovaObject,
to a new class NovaObjectDictCompat. All existing objects are
updated to add this new class as a mix-in parent.

In future any completely new objects (ie ones which aren't being
used to converted existing code from dict instances) should avoid
inheriting from NovaObjectDictCompat. Existing objects should
also have this parent class removed once all callers are audited
and/or updated to ensure they do not require dict compat.

Change-Id: I03f93f0c40df6f5f7df9cefe28dff900c22294c9
Merge "objects: allow creation of objects without dict item compat"