This change adds support for network configuration via systemd-networkd
on Ubuntu systems.

This is implemented via an Ansible Galaxy role,
stackhpc.systemd_networkd which was forked from
aruhier.systemd_networkd. Several improvements were made in
https://github.com/stackhpc/ansible-role-systemd-networkd/pull/1,
including:

* Add support for removing unexpected config files
* Use become where necessary
* Refactor config generation into a single task to improve performance

The systemd_networkd role does not add much abstraction on top of the
systemd-networkd configuration file format, which provides a lot of
flexibility at the expense of additional code in Kayobe. This code is
implemented as filter plugins, similarly to the existing
MichaelRigart.interfaces role.

This patch includes support for:

* Ethernet interfaces
* bridges
* bonds
* VLANs
* virtual Ethernet pairs (to connect Linux bridges and OVS bridges)
* static IP addresses
* static routes
* MTU

Some network attributes are currently not supported for
systemd-networkd:

* rules
* route options
* ethtool_opts
* zone
* allowed addresses

Story: 2004960
Task: 41881

Change-Id: I248b5bb9ce5a80a07a2a311cb3aca6daca920720

Issues:

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

Story: 2004960
Task: 41560

Change-Id: Ib124dc6f313a2a13fa509b10702c6141f102f2f9

* 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