Kubeadm: Automate certificate copy between control planes in HA setups

- Joining control plane nodes to a HA cluster can now be simplified by enabling the optional automatic copy of certificates from an existing control plane node.

* Fix a bug where soft eviction would not trigger when the threshold was crossed ([#52046](https://github.com/kubernetes/kubernetes/pull/52046), [@dashpole](https://github.com/dashpole))
* Addon manager supports HA masters. ([#55782](https://github.com/kubernetes/kubernetes/pull/55782), [@x13n](https://github.com/x13n))

1.6 encourages etcd3, and switching from etcd2 to etcd3 involves a full
migration of data between different storage engines.  You must stop the API
from writing to etcd during an etcd2 -> etcd3 migration.  HA installations cannot
be migrated at the current time using the official Kubernetes procedure.

1.6 will also default to protobuf encoding if using etcd3.  **This change is

the cluster lifecycle building block, continues to receive features and stability work required for bootstrapping production clusters efficiently. kubeadm has promoted high availability (HA) capability to beta, allowing users to use the familiar `kubeadm init` and `kubeadm join` commands to [configure and deploy an HA control plane](https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/high-availability/). An entire new test suite has been created specifically for ensuring these features...