We should install python3-* instead of python2-* in CentOS8.
This change fixes the remaining usage of python2-* left for
netwokring-ansible and networking-baremetal.
Backport note:
This includes 260aa9fe29 which fixed
the wrong package names in unit tests.
Change-Id: I136a81cc857130b3befa5df204525bfa6c6f0206
(cherry picked from commit 34b5036ed8)