refactor admission controller code

During the logging review in YUNIKORN-927 I noticed a possible nil pointer and a number of sub optimal code paths:

• in the serve function we check for a nil AdmissionReview.Request this is after all the calls are made that access the same object without nil checks. We either do not need this or have possible nil pointers
• we should unwrap the AdmissionReview and use the AdmissionRequest in our internal calls
• we create AdmissionResponse objects all over the code (9 places) we need to refactor that into a single function that takes the fields: uid, resultMessage and patch and returns the correct object