Each object with a list duplicated the _make_list() method in its
own module. This removes that duplication and adds a generalized
helper in objects/base.py. The instance object still uses its own
because it has to do a bunch of other stuff in the loop for
efficiency.
Change-Id: Ic910f39087ebc167b2b930979f7951116caf8598
Creates a new DBObjectMixin class for DB-backed objects to use. This
allows us to create non-DB-backed objects without having the common DB
fields automagically added to them.
Related to blueprint unified-object-model
Change-Id: Iaf2d8500505e9acdbffffb1d4bd3db0870ae82a3