Getting rke and Rancher setup to run kubernetes on arm is interesting. There is no official support yet via rancher, although there is interest and some work done towards those efforts. This is my attempt at getting a cluster of 3 Pis (2 3Bs and 1 3B+) provisioned and registered to a rancher 2 server.
I've successfully completed this both with Hypriot OS 1.9.0 and the arm64 builds https://github.com/DieterReuter/image-builder-rpi64 Both times I used the same basic cloud-init setup