Add support for Cilium as replacement CNI

[brumhard]

Cilium should be supported as replacement for the default flannel CNI. Docs on the K3s Cilium setup can be found here.

Requires #1 to disable flannel.

[brumhard]

There are two options for the implementation:

1. Installing and using cilium-cli on the master node. The problem with this is that the kubeconfig by default has 700 permissions and the cilium cli does not work with sudo (Error on cilium context when $KUBECONFIG requires root privileges cilium/cilium-cli#617)
2. Calling the cilium-cli install code from the pulumi provider. The problem with this is that the k8sclient implementation used for install.NewK8sInstaller is in an internal package and therefore not usable. Other than that the code is nicely reusable and separated.

[brumhard]

Opened an issue for 2nd option: cilium/cilium-cli#618