With the move to RHEL/CentOS 8 we no longer have Python 2 in our images
so there is no need for checking which Python version (2.x or 3.x) is
used inside of containers.
We also no longer have to support yum as a value for
distro_package_manager.
Partially-Implements: blueprint centos-rhel-8
Change-Id: Ie45cf3465fedddbde7856961527421883ba3d5c9
53443c5c71