public String flagValueForClusterType(StringFlag flag, Optional<ClusterSpec.Type> clusterType) {
            return clusterType.map(type -> flag.with(CLUSTER_TYPE, type.name()))
                              .orElse(flag)
                              .value();
        }
Returns the host provisioner to use, or empty to use the default provisioner,
creating hosts from the application package defined hosts
configserver/src/main/java/com/yahoo/vespa/config/server/deploy/ModelContextImpl.java