private int totalAllocatedTo(ClusterSpec cluster) {
        int count = 0;
        for (Map.Entry<ClusterSpec, List<HostSpec>> allocation : allocations.entrySet()) {
            if ( ! allocation.getKey().type().equals(cluster.type())) continue;
            if ( ! allocation.getKey().id().equals(cluster.id())) continue;
            count += allocation.getValue().size();
        }
        return count;
    }
Create a new provisioned instance to record provision requests to this and returns it
config-model/src/main/java/com/yahoo/config/model/provision/InMemoryProvisioner.java