static boolean zoneIsStable(NodeList allNodes) {
        return allNodes.state(Node.State.active).stream()
                       .noneMatch(node -> node.allocation().get().membership().retired() ||
                                          node.status().wantToRetire() ||
                                          node.status().preferToRetire());
    }
Returns true if no active nodes are retiring or about to be retired
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/NodeMover.java