@Override
        public int compareTo(HostCapacity.NodeWithHostResources b) {
            int result = compare(b.hostResources, hostResources);
            if (result != 0) return result;
            return b.numFreeIps - numFreeIps;
        }
Spare hosts are the hosts in the system with the most free capacity. A zone may reserve a minimum number of spare
hosts to increase the chances of having replacements for failed nodes.

We do not count retired or inactive nodes as used capacity (as they could have been
moved to create space for the spare node in the first place).

@param candidates the candidates to consider. This list may contain all kinds of nodes.
@param count the max number of spare hosts to return
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/provisioning/HostCapacity.java