Nunux cloud configuration website.
Go to ROM-o-matic website in order to generate iPXE image.
Choose "ISO bootable image" as output format and paste the following script:
#!ipxe
set base-url http://stable.release.core-os.net/amd64-usr/current
set config-url http://ncarlier.github.io/nunux-cloudinit/conf/nunux.yml
kernel ${base-url}/coreos_production_pxe.vmlinuz console=tty0 rootfstype=btrfs cloud-config-url=${config-url}
initrd ${base-url}/coreos_production_pxe_image.cpio.gz
boot
Build and upload the image to your cloud provider. Finally boot the machine.