public List<Node> upgradeOs(Predicate<Node> filter, Optional<Version> version) {
        return performOn(filter, (node, lock) -> write(node.withWantedOsVersion(version), lock));
    }
Set target OS version of all nodes matching given filter.

@return the nodes in their new state
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/node/Nodes.java