Sync module units from oslo

Module units landed in Nova first, then was accepted by oslo with
minor changes. This patch removes nova specific unit module and
test, replaces with module units in oslo.

commit in Oslo:'Add unit constants' f9308560c36918cc

Change-Id: I6054c18a56f59d33a69f4714be19d34fbe9de335
Co-Authored-By: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
Merge "Sync module units from oslo"