Simplify Process Service Delegate for REST endpoints

The process service delegate was created to translate to simple Java methods all the logic that is in REST services.

During the development of the stateful process API it was evidenced that there are some methods (abortTask() and completeTask()) that could be easily redefined in terms of one (transitionTask()) 

Moreover, the pair <user,groups> can be substituted for a Policy object.

These are all enhancement that are useful for realizing the Public API