document VM testing procedure

[ktdreyer]

Here is how I tested changes to the testnodes role recently in a VM. It's a little different than the README.rst (that instructs users to edit files in /etc/ansible, for example)

1. Create a new CentOS or RHEL VM on my laptop
2. Create a local hosts file with the IP of my VM, like:

   [testnodes]
   192.168.122.31 ansible_user=centos ansible_ssh_extra_args="-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null"
   

3. To debug SSH connections, try running Ansible's setup module, like:

   ansible testnodes -i hosts -m ansible.builtin.setup | less
   

4. Finally, run the playbook on the VM:

   ansible-playbook -i hosts -v testnodes.yml
   

... then examine the files or features on the VM to make sure they do what I expect.

We could edit README to make it simpler and match this procedure.