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Support Ubuntu 20.04

The major change here is that Ubuntu 20.04 moves the default root from
/var/lib/tftpboot to /srv/tftp. That matches what Debian does. The data
files are changed so that only Ubuntu 16.04 and 18.04 have overrides.
This technically breaks versions in between, but those aren't supported.

Update the supported operating systems

Adds Red Hat 8, CentOS 8, Fedora 30 and 31.

This also changes the default for RH-based distros to use a daemon
instead of xinetd while keeping xinetd for EL 6 and 7.

drop EOL OSes

Fixes #25190 - Allow overriding the tftp map source

add option to manage root_dir

Set the service for the Red Hat osfamily

This allows setting $daemon to true on Red Hat systems with systemd.
Because the migration is hard we don't flip the value.

Convert params pattern to module data with Hiera 4