slapos.slap: Reorganise Hateaos code

  Move Hateoas related code from core API to an separated file slapos.slap.hateoas which can be reusable.
  Move exceptions to slapos.slap.exceptions to be able to use on both files
  Move Hateoas code from https://lab.nexedi.com/nexedi/slapos.toolbox/blob/master/slapos/agent/tester.py into slapos core to make it reusable.