Generalize the _make_list() function for objects
Generalize the _make_list() function for objects

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

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects