Update scenarios

Related to https://github.com/KanoComputing/peldins/issues/735

This PR contains the redesigned post- and pre- upgrade processes. It also fixes some problems we've been having with the version during image build (issue https://github.com/KanoComputing/peldins/issues/700). Basically, if there's no version file on the system, the updater will initialise it as if the system were up-to-date and it won't run any pre or post updates on it.

cc @alex5imon