dracut-reencrypt: add finished hook

Closed username-removed-1275695 requested to merge haraldh/cryptsetup:finished into master

A finished hook prevents dracut-initqueue from exiting and lets it finish the batched jobs. Without a "finished hook" and without "root=" on the kernel command line, the reencrypt job would not be executed.

Normally you want to reencrypt without a "root=" on the kernel command and want to reboot after the reencrypt job is done.

This patch adds the missing "finished hook".