Comment 2 for bug 1571761

So:

a.) zfs-import-cache.service is 'After systemd-udev-settle.service'
b.) cloud-init installs /lib/udev/rules.d/79-cloud-init-net-wait.rules which uses IMPORT of /lib/udev/cloud-init-wait to block network device add events until cloud-init-local has written networking configuration

I'm pretty sure, then what is happening is:
1.) network device gets added and cloud-init-wait starts its 60 second wait for cloud-init-local to finish.
2.) systemd-udev-settle runs, which waits until queued all events are processed
3.) deadlock until timeout from 1