Create a jobset concept

[JulienMalka]

Currently we only have a way to upload already evaluated trees that can define reports. The problem is that these report stay static in time, a new report has to be uploaded to define a new version of the minimal iso image, for example.

We need to be able to instead define "jobsets", that is a nix expression that periodically gets evaluated to update a report.

[raboof]

:+1: - rather than 'updating' a report perhaps we should allow/create a number of reports for the same jobset over time, so we can compare them