// admitPod makes sure a new pod does not set spec.Priority field. It also makes sure that the PriorityClassName exists if it is provided and resolves the pod priority from the PriorityClassName.
// Note that pod validation mechanism prevents update of a pod priority.
// admitPriorityClass ensures that the value field is not larger than the highest user definable priority. If the GlobalDefault is set, it ensures that there is no other PriorityClass whose GlobalDefault is set.