Add keyslug testing capability to behat tests

[phavekes]

This issue is imported from pivotal - Originaly created at Mar 27, 2019 by Michiel Kodde

In this PR we added support for overriding the desired signing key id (keyslugging). This feature is currently nog tested in the Behat test suites.

This would be a valuable addition.

[baszoetekouw]

This is becoming more urgent, as we are going to do a key rollover (with a new key slug) next year. Would be very welcome if this functionality is fully tested.

[johanib]

I'm not 100% sure why we put assigned this to Bas, but added a test check the rollover key is used if the rollover request is used in #1991