Test LUKS device reuse

[bgilbert]

We don't currently have tests for reusing a LUKS device. kola tests can't do this, since they can't reuse a secondary disk with a fresh boot disk. The blackbox tests have a couple tests for filesystem reuse, but don't understand LUKS at all.

This contributed to the regression in #1305.

[jlebon]

We don't currently have tests for reusing a LUKS device. kola tests can't do this, since they can't reuse a secondary disk with a fresh boot disk.

Hmm, tricky. Was thinking of different ways we could support this in kola, but in the end it's probably better long-term to teach LUKS this to the blackbox tests.

[jlebon]

coreos/coreos-installer#712 could be used for this, though it would be inefficient.