Provide backwards-compat env variables
Provide backwards-compat env variables

These have been used by serverextensions (like nbinteract) and
possibly by other users in various places. DockerSpawner also
sets these. Let's keep these for now, but plan to deprecate them
later.

yuvipanda/jupyterhub-kubernetes-spawner,jupyterhub/kubespawner,ktong/kubespawner,jbmarcille/kubespawner