Further fixes for 2.0

[stefanlasiewski]

I'm contributing a few additional fixes for 2.0. They are summarized as:

• Update the version of RancherOS
• Use the :stable branch of the rancher/rancher image
• Enable the use of $kubernetes_version so we can play with newer version of k8s.
• Remove some kruft from the code
• Update the README.

This project works. I've been using it on and off for 4 weeks, and it also works for my colleague.

[cjellick]

Closing this due to staleness. Feel free to reopen or open a new PR if there's still a relevant change to be made. Thanks!