Avoid potential loop_var shadowing (#11162)

With CentOS, kubespray currently produces the following warning:

[WARNING]: TASK: bootstrap-os : Enable Oracle Linux repo: The loop variable
'item' is already in use. You should set the `loop_var` value in the
`loop_control` option for the task to something else to avoid variable
collisions and unexpected behavior.

This could bites us in nasty ways, so fix it.