private Set<String> hostsOf(NodeList nodes, Optional<ClusterSpec.Type> clusterType) {
        return nodes.asList().stream()
                    .filter(node -> clusterType.isEmpty() ||
                                    clusterType.get() == node.allocation().get().membership().cluster().type())
                    .flatMap(node -> node.parentHostname().stream())
                    .collect(Collectors.toSet());
    }
Non-exclusive app first, then an exclusive: Should give the same result as above
node-repository/src/test/java/com/yahoo/vespa/hosted/provision/provisioning/VirtualNodeProvisioningTest.java