Debian Buster support

In case people are wondering, I’ve started Tracker #27282: Debian Buster support - Installer - Foreman. This is to run on Buster.

Note there are still some things to work out.

For example, makes proper support in forklift harder because the Vagrant image is still pre-release. We can work around this in our update role, but it’d be much easier to wait.

Puppetlabs hasn’t built Puppet for Buster but the native OS package is at least Puppet 5 so that allows some initial work.