Revert "Change cgroup driver to systemd (#521)"

[abeer91]

This reverts commit f9d2c39.

Description of changes:

EksCtl uses a nodebootsrap process where it configures the kubelet config, but not the docker config

Before we revert this commit, we need to finalize on how we can safely update eksctl to also set these values.

https://github.com/weaveworks/eksctl/blob/1555c35b1b315ff31c062a789c5f756a6122d057/docs/current-001-node_bootstrap.md

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[reegnz]

I don't necessarily understand why a revert. A tool downstream is breaking, why should that influence what happens in the upstream?

[reegnz]

@abeer91 Also could you please link to the ticket in eksctl that tracks this issue, so we can re-apply the change once the downstream manages to fix their incompatibility?