public boolean isWorking() {
        NodeList activeNodes = list(Node.State.active);
        if (activeNodes.size() < 20) return true; // Not enough data to decide
        NodeList downNodes = activeNodes.down();
        return ! ( (double)downNodes.size() / (double)activeNodes.size() > 0.2 );
    }
Returns whether the zone managed by this node repository seems to be working.
If too many nodes are not responding, there is probably some zone-wide issue
and we should probably refrain from making changes to it.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/node/Nodes.java