Replace the base image of jovial/jovialuser

[capgadsx]

Currently we use jupyterhub/singleuser:0.8, wich contains many things that we don't use, like anaconda.
We should change it so our jovialuser image use less space

[maray]

Wait... we REALLY use anaconda. That is the whole idea of the project. Please specify a little more what we need to prune, because I also agree that we need to keep the image as shallow as possible.