@InterfaceAudience.Private
    public RegionMoverBuilder rackManager(RackManager rackManager) {
      this.rackManager = rackManager;
      return this;
    }
Set specific rackManager implementation. This setter method is for testing purpose only.
@param rackManager rackManager impl
@return RegionMoverBuilder object
hbase-server/src/main/java/org/apache/hadoop/hbase/util/RegionMover.java