ansible.cfg: Use auto discovery of Python interpreter

Uses a built-in table of OSes and Python versions to decide which
Python interpreter to use. This is better than hard coding python3
in every single host's host_vars.

See: https://docs.ansible.com/ansible/latest/reference_appendices/interpreter_discovery.html

ansible.cfg

@@ -5,6 +5,7 @@ inventory=hosts
 # instead of using --ask-vault-pass
 ask_vault_pass=True
 remote_user = provisioning
+interpreter_python=auto
 
 ansible_managed = This file is managed by Ansible.%n
   template: {file}