os-apply-config template of etc/os-net-config/config.json is created in tripleo-image-elements. This file has to be provided by package instead of manually creating it.
Change to install os-net-config as package, not element -https://review.openstack.org/#/c/470017/
Doc changes are here: https://review.openstack.org/#/c/521197/
os-apply-config is no longer used as of OSP-12. I'm closing this out, since os-net-config will not be supplying it.