RFC/PREP: Allow a load-balancer address for k8s API

[rowanj]

Extends the RancherKubernetesEngineConfig (cluster.yml) type to support the configuration of an API load-balancer.

This is a pre-requisite for an upcoming PR to extend RKE to support such a configuration.

to-do:

• update and document design in accordance w/ feedback from #1348

--

This optional string is to replace the hostname used by other nodes and
configurations when attempting contacting the k8s API service.

It is expected to be the hostname of a layer-4 (TCP) load-balancer which
will be externally configured by the organisation to direct traffic to
an appropriate selection of the cluster's ControlPlane nodes on the same
port as it arrived; i.e. TCP/6443 for kube-apiserver.

Any TLS configuration and healthchecks on the load-balancer host are
also expected to be managed by the organisation.

[superseb]

The design for a solution for this has been posted at rancher/rke#1348 (comment) and will be put in a final design soon that can be worked on. Is this taken into account or is this an implementation that you thought of? And is that design described somewhere?

[rowanj]

Need to regenerate the built code but the build seems to be failing here due to our intercepting proxy; will continue from home.

[rowanj]

@superseb I didn't see that PR in my search, this is based on our current need

[rowanj]

I like most of the suggested design there, I'll adapt and extend and update this PR.

[superseb]

There is now accepted design yet, I just opened rancher/rke#1682, will have it reviewed and we can work from there.