move audit-related code to backend

Add two functions in filebackend, to retrieve all audits, and to
retrieve an audit script's config given the name.

Replace occurences of utils.load_yaml in engine.py with calls to
above functions

Change-Id: Ia204d0db8a078c28ce27a2ae2ed57eb8b7fdf23b
Merge "move audit-related code to backend"