Support running without k8s for BinderHub

[yuvipanda]

From https://discourse.pangeo.io/t/notes-from-the-pangeo-2i2c-kick-off-meeting/1587,

Remove artificial distinctions between BinderHub/JupyterHub. Currently BinderHub and JupyterHub have strong distinctions between them, but this is not strictly necessary. Binder-like functionality should be a feature flag on any JupyterHub, and thus BinderHub could cease to be a standalone tool, and instead be a “particular configuration of a JupyterHub”.

Part of this is to disentangle binderhub from the current setup wrt kubernetes - particularly the helm chart. Binderhub's architecture makes this quite possible.

jupyterhub/binderhub#1318 will be a start, making kubernetes a soft dependency of binderhub. This will also help extract functionality that's in the helm chart that doesn't need to be.