Default Plasma config should not use Named servers

[Pierre Poulain]

Jupyer Hub named servers are very powerful but could be puzzling for users.

Plasma default should stick to only one default server. Of course, named servers can be activated later.

This new default will dramatically simplify Hub Control Panel going from:

to only:

Issue #130 could be reactivated once we will be able to simplify the Named servers UI and maybe have only servers named after environment names.