@Override
    public void activate(Collection<HostSpec> hosts, ActivationContext context, ApplicationTransaction transaction) {
        validate(hosts);
        activator.activate(hosts, context, transaction);
    }
Returns a list of nodes in the prepared or active state, matching the given constraints.
The nodes are ordered by increasing index number.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/provisioning/NodeRepositoryProvisioner.java