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

(As decided on #12299 (closed).) The wrap_test_suite script shall default to passing the previous release to --old-iso unless the kill switch is enabled. Implementation suggestions: #12299-note_1.

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

Related issues

Blocked by #14608 (closed)
Blocks #14611 (closed)

Original created by @intrigeri on 14609 (Redmine)

Edited Nov 22, 2019 by intrigeri

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