let user specify custom endpoint / path when using custom images

[cboettig]

When a user brings a custom image (docker image or repo2docker-style github repo) they should be able to specify the default endpoint (e.g. /lab, /rstudio etc). Alternately/meanwhile could /lab just be the default endpoint? It currently seems to default to /tree, which makes sense for historic compatibility but is confusing.

[batpad]

This should be close-to-possible with the latest changes - I think we're very close to having any kubespawner over-rides be setup in the profile and have it be rendered in the form and this is a very good use-case to test :-)

Let me put some thought into this and see how easy / hard this is to do, but enabling things like this is definitely the idea with the current changes.

Thanks for the ticket @cboettig !