Don't pass --old-iso when testing feature/buster*
Originally created by @anonym on #16754 (Redmine)
Our test suite does not support 3.x (stretch) and 4.x (buster) at the same time (that would need extra work, e.g. keeping 3.x versions of a few images. For now, let’s just not pass —old-iso at all; once that works we could consider adding support for a 3.x —old-iso.
Feature Branch: puppet-tails:bugfix/16754-dont-use-old-iso-when-testing-buster-images