Migrate Saltstack to 'onedir' repo

[Kurocon]

Saltstack decided that their old way of packaging was not good enough any more, and now has a new repository with a new package format. Unfortunately they are not updating the old repos any more, so we are stuck on version 3005 until we update.

The pain point of this migration is that old minions won't work with the newer style master, so the upgrade should ideally be done in one go. We first need to check if the Debian versions we use are all supported on the new repo though.

The upgrade guide is here:
https://docs.saltproject.io/salt/install-guide/en/latest/topics/upgrade-to-onedir.html