Issues:

* bond configuration not working on Ubuntu - disabled in CI test

Story: 2004960
Task: 41560

Change-Id: Ib124dc6f313a2a13fa509b10702c6141f102f2f9

The flag dnf_install_epel was being used to predicate installation
of the EPEL RPM repo package, but not preventing the installation
of configuration for access to EPEL.  This small patch completes
the job.

Updates the overcloud host configure CI job to install EPEL mirrors,
since it was previously relying on this buggy behaviour.

Change-Id: Ib417837d0772338b16ea9f7f2540549f277d5de8

Change-Id: Icb57b908be9c83700274c042185e4481d5ca6ea5

This gives us some test coverage with Ansible Vault.

Change-Id: Ic9db7f84e9f0dcbe69c31b6fff6bda3dfb401a33
Story: 2004959
Task: 39813

* 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