Kubernetes level cluster health checks

[jefflill]

ClusterProxy.GetHealthAsync() is currently doing mostly hosting level cluster health checks. We need to enhance this by using the API server's /livez and /readyz endpoints.

https://kubernetes.io/docs/reference/using-api/health-checks/

We're already calling this in ClusterProxy.GetDeploymentAsync() to avoid a long retry period when the cluster isn't running. We need to make this more comprehensive.