Update chapter 4 kubernetes cluster build instructions to use 1.17

Chapter 4's "Building a Kubernetes cluster with Ansible" currently installs Kubernetes 1.16. This should be updated to use the latest version of Kubernetes (currently 1.17... could be 1.18 soon).

This could require a little coordination with the `geerlingguy.kubernetes` role if there are any breaking changes—see https://github.com/geerlingguy/ansible-role-kubernetes