private void upgradeFlavor(Node host, boolean upgrade) {
        nodeRepository().nodes().upgradeFlavor(host.hostname(),
                                               Agent.HostFlavorUpgrader,
                                               nodeRepository().clock().instant(),
                                               upgrade);
    }
This maintainer attempts to upgrade a single host running on an older flavor generation. The upgrade happens by
marking and retiring the host on the old generation, and redeploying to provision a replacement host on a newer
generation.

If the cloud provider reports a lack of capacity for the newer generation, retirement of the host is
cancelled, and upgrade is attempted of the next host on an old flavor, if any.

Once a host has been marked for upgrade, {@link HostResumeProvisioner} will complete provisioning of the replacement
host.

@author mpolden
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/HostFlavorUpgrader.java