Update to Kubernetes 1.18.6

[aleks-mariusz]

This PR addresses #15 and makes the following version changes:

• Kubernetes 1.17.2 -> 1.18.6
• cri-tools 1.17.0 -> 1.18.0
• containerd 1.3.2 -> 1.3.6
• CNI plugins 0.8.5 -> 0.8.6
• etcd 3.3.18 -> 3.4.10
• coredns 1.6.3 -> 1.7.0
• cfssl/cfssljson 1.2 -> 1.4.1
• ubuntu 18.04 lts (bionic) -> 20.04 lts (focal)

The above represent the same version changes in the "upstream" repo this repo is meant to be based on.

Additionally, the following changes were also needed:

• kube-apiserver systemd unit file had one change relating to the --runtime-config parameter needing to be in quotes (per the same upstream change)
• kube-controller-manager --address parameter changed to --bind-address.
• this version of etcd requires more strict permissions on /var/lib/etcd to run (otherwise it refuses to start).
• lastly, the containerd extraction was clobbering the /bin symlink to /usr/bin which which was causing strange puzzling issues.. i've changed this to match along the lines of what Kelsey does now