ci: Format ansible-playbook output as yaml

Author: moreati

tests/ansible/ansible.cfg

@@ -5,7 +5,9 @@ strategy_plugins = ../../ansible_mitogen/plugins/strategy
 inventory_plugins = lib/inventory
 action_plugins = lib/action
 callback_plugins = lib/callback
-stdout_callback = nice_stdout
+stdout_callback = yaml
+stdout_whitelist =
+    yaml
 vars_plugins = lib/vars
 library = lib/modules
 filter_plugins = lib/filters