* Always use Python 3
* Drop code paths for CentOS 7
* Drop support for Yum
* Remove support for host NTP daemon, always use chrony
* Switch references from 'yum_install_epel' to 'dnf_install_epel'
* Remove overcloud host image workaround for tagged VLAN admin network
* Remove the kayobe.utils.yum_install function, which is unused

Change-Id: I368f6edafed9779658798fc342116b4c1b3ffd48
Story: 2006574
Task: 39481

This can be advantageous in deployments with a data security
requirement.

Change-Id: I555ee575ccec0cfbcc4c4bcb53677796c83227e3
Story: 2007555
Task: 39410

Tests various non-default configuration:

* Custom users
* Network interfaces, VLANs, bridges, bonds
* Software RAID
* LVM & docker devicemapper
* timezone
* Package mirrors
* yum-cron / DNF automatic

This improved test coverage allows us to be more confident about these
features working on CentOS 8.

Change-Id: I36148e4356deb7d5ec00d8d3ebeb2d3932ff4f94
Story: 2006574
Task: 38938