@param queueFairShare fairshare in the queue
   * @param queueUsage resources used in the queue
   * @param maxAvailable available resource in cluster for this queue
   * @return calculated headroom
@param queueFairShare fairshare in the queue
   * @param queueUsage resources used in the queue
   * @param maxAvailable available resource in cluster for this queue
   * @return calculated headroom | SchedulingPolicy.java