Add support for a external etcd cluster

[xunholy]

https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/setup-ha-etcd-with-kubeadm/

[crutonjohn]

just for clarification on this one: do we want to assist the user in setting up an external etcd cluster, or just accept parameters for connecting to an external etcd?

[RobReus]

I would say both. But I would aim this issue towards implementing the parameters required for this. The parameters can then later be used as well when we make it possible to setup an etcd cluster within ansible too.

We can then create a new issue to add the actual setup of an etcd cluster to the ansible files at a later point. This because setting up a proper etcd cluster is not as trivial as installing some packages and be done with it.

[crutonjohn]

setting up an external etcd cluster is out of scope for this project, but supporting users who run an external etcd cluster is something we can move to support.