fix(dracut-shutdown): add cleanup handler on failure

fix(dracut-shutdown): add cleanup handler on failure

It may happen that dracut-shutdown.service fails, for example on timeout
due to very low bandwidth.
In such case, for hardening purposes, a new dracut-shutdown-onfailure.service
unit doing dracut-shutdown.service cleanup needs to execute to make sure
switching root to an incomplete initramfs won't occur later.

See also RHBZ #1924587 (https://bugzilla.redhat.com/show_bug.cgi?id=1924587).