Get headroom by calculating the min of <code>clusterAvailable</code> and
   * (<code>queueFairShare</code> - <code>queueUsage</code>) resources that are
   * applicable to this policy. For eg if only memory then leave other
   * resources such as CPU to same as clusterAvailable.
Get headroom by calculating the min of <code>clusterAvailable</code> and
   * (<code>queueFairShare</code> - <code>queueUsage</code>) resources that are
   * applicable to this policy. For eg if only memory then leave other
   * resources such as CPU to same as clusterAvailable. | SchedulingPolicy.java