Please note that this MR needs this slapos.toolbox MR : slapos.toolbox!43 (merged)

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 )