Support for Cilium as network-plugin

[runlevel-six]

What would it take to get support for Cilium as a network-plugin instead of just Calico? Is this something we can easily do on our own? I'm not seeing an easy way to customize the CAPI templates used with the Magnum/ClusterAPI driver (primarily the pre/post kubeadm command sections in the kubeadmcontrolplane objects).

[okozachenko1203]

• support overrides of magnum api validator at the driver level https://review.opendev.org/c/openstack/magnum/+/904835 https://review.opendev.org/c/openstack/magnum/+/905427
• implement cilium cni in magnum-cluster-api-driver feat: support cilium cni #287