Launch a Type-0, Type-1 or Type-3 Ubuntu 14.04 server and use the below cloud config. You can add any additional RancherOS configuration to it, but below is the bare minimum you need to provision RancherOS.
#cloud-config
runcmd:
- wget -O /tmp/cc https://raw.githubusercontent.com/rancher/os/master/scripts/hosting/packet/packet.sh
- bash -x /tmp/cc