Use shared-secrets.d instead of TailsToaster_config/common.d

Originally created by @anonym on #11801 (Redmine)

We use shared-secrets.d when developing tests, so our isotesters should use it as well so developers can push updates required for new tests that will go live on the isotesters as well.

This would also have the advantage of making tails::jenkins::slave::iso_tester work a bit more out-of-the-box on other systems than lizard: currently, systems managed by that class get configuration (e.g. ssh.yml) that can’t possibly work outside of lizard.

Related issues

• Has duplicate tails/tails#12559