@Override
  void prepare(final BalancerClusterState cluster) {
    this.cluster = cluster;
    this.loadRules();
  }
Called once per LB invocation to give the cost function to initialize it's state, and perform
any costly calculation.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/HeterogeneousRegionCountCostFunction.java