[cern] Add option to specify Cgroup driver for Kubelet

upstream: https://review.openstack.org/#/c/571583/

This patch allows specification of Cgroup driver for Kubelet service. The necessity of this patch was realised after upgrading Docker to the new community edition (17.3+) which defaults to cgroupfs Cgroup driver but on the other hand, Fedora Atomic (version 27) comes with 1.13. Cgroup drivers for Docker need to be identical for the two services, Docker and Kubelet, need to be able to work together.

Story: 2002533 Task: 22079 Change-Id: Ia4b38a63ede59e18c8edb01e93acbb66f1e0b0e4

Conflicts: magnum/drivers/common/templates/kubernetes/fragments/configure-kubernetes-master.sh Drop changes for master node. In master kubelet runs on the master nodes too.

Closes OS-6955