private static ClusterSpec asSpec(Optional<String> clusterType, int index) {
        return ClusterSpec.request(clusterType.map(ClusterSpec.Type::from).orElse(ClusterSpec.Type.content),
                                   ClusterSpec.Id.from(String.valueOf(index)))
                          .vespaVersion(Vtag.currentVersion) // Needed, but should not be used here.
                          .build();
    }
Try to allocate the preprovision cluster capacity.

@param mutableNodes represents all nodes in the node repo.  As preprovision capacity is virtually allocated
                    they are added to {@code mutableNodes}
@return the part of a cluster capacity it was unable to allocate, if any
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/HostCapacityMaintainer.java