code: public Vertigo deployCluster(String cluster, String group, int nodes) {
      return deployCluster(cluster, group, nodes, null);
    }
docs: Deploys multiple cluster nodes to a specific cluster group.
  
  @param cluster The cluster event bus address.
  @param group The cluster group to which to deploy the nodes.
  @param nodes The number of nodes to deploy.
  @return The Vertigo instance.