RKE system-images docs are out of date, confusing, and subomptimal

[cjellick]

This doc page https://rancher.com/docs/rke/latest/en/config-options/system-images/ has a few problems.

1. The kubernetes version we use as an example is v1.10.3-rancher2 is comically old. We should update

2. The docs link out to https://github.com/rancher/kontainer-driver-metadata/blob/master/rke/k8s_rke_system_images.go. This link is confusing and misleading because the images on this page are not real. For example quay.io/coreos/flannel:v0.11.0-rancher1 is an upstream repo and a rancher tag. That is a made up combination that we apparently do post-processing on later to turn into `rancher/coreos-flannel:v0.11.0-rancher1. A better alternative is the list of images in their final state: https://github.com/rancher/kontainer-driver-metadata/blob/master/data/data.json#L3816-L3847

3. The rke cli has a command for getting this data rke config --system-images. This should probably be the PRIMARY way we direct people to get lists of rke images.

[SheilaghM]

recommend 1.20 k8s version
Have Craig review the PR