Remove aufs-tools from Ubuntu requirement (#8754)

aufs-tools was required for docker.io package originally,
but Kubespray installs docker-ce package instead today.
In addition, Ubuntu 20.04 doesn't provide aufs-tools as [1].
Then this removes aufs-tools from Ubuntu requirement.

[1]: https://bugs.launchpad.net/ubuntu/+source/aufs-tools/+bug/1947004