Provide mechanism for administrator to place a custom
key for potential interactive recovery into
/var/lib/confluent/private/os/<profile>/pending/luks.key
If not provided, generate a unique one for each install.
Either way, persist the key in /etc/confluent/luks.key, to
facilitate later resealing if the user wants (clevis nor systemd
prior to 256 supports unlock via TPM2, so keyfile is required
for now).
Migrating to otherwise escrowed passphrases and/or sealing to
specific TPMs will be left to operators and/or third parties.