Add support for --disable-apiserver --disable-controller-manager --disable-scheduler (to dedicate some vms to run etcd)

[gberche-orange]

As a k3s operator
In order to run etcd on dedicated k3s-server bosh vms
I need these nodes to not be running other k8s components

See https://docs.k3s.io/installation/server-roles#dedicated-etcd-nodes

Dedicated etcd Nodes

To create a server with only the etcd role, start K3s with all the control-plane components disabled:

server --cluster-init --disable-apiserver --disable-controller-manager --disable-scheduler

This first node will start etcd, and wait for additional etcd and/or control-plane nodes to join. The cluster will not be usable until you join an additional server with the control-plane components enabled.

Dedicated control-plane Nodes
note: A dedicated control-plane node cannot be the first server in the cluster; there must be an existing node with the etcd role before joining dedicated control-plane nodes.

To create a server with only the control-plane role, start k3s with etcd disabled: sh -s - server --token <token> --disable-etcd --server https://<etcd-only-node>:6443