Revert "AMBARI-5199. Replace $.ajax with App.ajax anywhere where it's possible. (onechiporenko)"

This reverts commit cf332bb382b752f1ba554e472545046d1458b73b.

Conflicts:
	ambari-web/app/controllers/wizard/step8_controller.js
AMBARI-5209. MapReduce2 Client Install fails after cluster upgrade. (swagle)
AMBARI-5216. stale_configs parameter works not properly after removing config group. (swagle)