private int nodesToRetire() {
        return Math.max(0, from.nodes() - to.nodes());
    }
Returns the estimated number of nodes that will be retired by this change,
given that it is a content cluster and no node replacement is necessary.
This is not necessarily always perfectly correct if this changes group layout.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/ResourceChange.java