Move the --old-iso kill switch from puppet-tails.git to jenkins-jobs.git

Originally created by @intrigeri on #14609 (Redmine)

(As decided on tails/tails#12299.) The wrap_test_suite script shall default to passing the previous release to --old-iso unless the kill switch is enabled. Implementation suggestions: https://gitlab.tails.boum.org/tails/tails/-/issues/12299#note_48293.

As part of this, document how to use the kill switch in a place where anonym may find it in N months.

Edited May 15, 2020 by intrigeri