Reintroduce the old_iso logic for release branches

Originally created by @intrigeri on #10409 (Redmine)

i.e. revert commit c1a1f7f0c9407490dc95784f625d33b54a71f4e8 in puppet-tails.

And while we’re at it 638798d69346619c9da0cc206a3d4b9f7a9e4ff will want a revert as well :)

Feature Branch: puppet-tails:bugfix/10409-reintroduce-old-iso-logic

Edited May 15, 2020 by intrigeri

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