@Override
    public String toString() {
        return "hosts [" + hostname2host.values().stream()
                                                 .map(HostResource::getHostname)
                                                 .collect(Collectors.joining(", ")) +
               "]";
    }
The parent node for all Host instances, and thus accessible
to enable services to get their Host.

@author gjoranv
config-model/src/main/java/com/yahoo/vespa/model/HostSystem.java