Please note that this MR needs this slapos.toolbox MR : https://lab.nexedi.com/nexedi/slapos.toolbox/merge_requests/43
I suppose also that we will need to update version of slapos.toolbox used by backupserver but I don't know how to do this (I have a local patch with develop = ..../slapos.toolbox).
As this is my first real MR for buildout, please fire all your comments ! (ping @vpelletier @jerome @Nicolas )