For privatestorageops#314 I wanted to find out how much user cipher text we are currently storing (it's around 50 GB per production storage node, however storage003 is at only 2 GB).
We don't have a graph that shows that yet, so this MR adds one.
I think this is useful as a proxy of system usage and thus "success", i.e. privatestorageops#12.
And it will tell us when to provision more storage (currently a rather early alert is set at 50 % usage, since ZFS doesn't tend well to being actually full is what I heard).
Also, this MR removes some "Threshold lines" since I found them misleading: A line just like the data lines directly at the threshold. Tzzz.
Instead, latest Grafana gives the option of just coloring the area. Let's do that instead.
