Change SystemUserSpawner to support new function def.
Change SystemUserSpawner to support new function def.

jlamcanopy/dockerspawner,jupyter/dockerspawner,ibab/dockerspawner,bigzachattack/dockerspawner,Xarthisius/dockerspawner,bigzachattack/dockerspawner,anaderi/dockerspawner,jhamrick/dockerspawner,minrk/dockerspawner,betatim/dockerspawner,Xarthisius/dockerspawner,betatim/dockerspawner,quantopian/dockerspawner,quantopian/dockerspawner,jhamrick/dockerspawner,Fokko/dockerspawner,jlamcanopy/dockerspawner,anaderi/dockerspawner,ibab/dockerspawner,minrk/dockerspawner,jupyter/dockerspawner,Fokko/dockerspawner