feat: add undef hints for runner variables

Author: jackhodgkiss

etc/kayobe/inventory/group_vars/github-runners/runners.yml

@@ -1,7 +1,7 @@
 ---
 runner_user: "{{ kayobe_ansible_user }}"
-github_account: stackhpc
-github_repo: stackhpc-kayobe-config
+github_account: "{{ undef(hint='Name of customer's GitHub account') }}"
+github_repo: "{{ undef(hint='Name of customer's kayobe config repository') }}"
 access_token: "{{ secrets_github_access_token }}"
 
 base_runner_dir: /opt/actions-runner