ansible: add list groups in an inventory example (#5755)

pages/common/ansible.md

@@ -27,3 +27,7 @@
 - Execute a command using a custom inventory file:
 
 `ansible {{group}} -i {{inventory_file}} -m command -a '{{my_command}}'`
+
+- List the groups in an inventory:
+
+`ansible localhost -m debug -a '{{var=groups.keys()}}'`