kubernetes: add cluster autoscaler config

[dhij]

This PR allows users to specify cluster autoscaler configuration in godo during kubernetes create and update. We accept a JSON string as the value for cluster autoscaler configuration.

[andrewsomething]

This is type string, but in the tests I see that is is a JSON object:

{"scale-down-utilization-threshold":"0.5","scale-down-unneeded-time":"1m30s"}

Is there any reason why doesn't use a more specific type? Generally, I'd expect something like:

type KubernetesAutoscalerConfiguration struct {
        ScaleDownUtilizationThreshold float64 `json:"scale_down_utilization_threshold"`
        ScaleDownUnneededTime         time.Duration `json:"scale_down_unneeded_time"`
}

[dhij]

This is a JSON string and I actually had the same question on this but apparently this is what Timo had left on the ticket: Given we may want to support more parameters over time, it is probably advisable to accept a generic JSON object with CA flags/values as key/value pairs. That minimizes maintenance effort and also allows us to (de-)serialize the field more easily in the k8saas DB.