add munin graphs of garbage percentage

[warner]

We have a few graphs right now that show the total deep-size number as it changes over time. We also have 'df' values that indicate how much of the actual disk is being used.

We should make a graph that shows the ratio of these two numbers. At the moment we think that about 30% of the data on our storage servers is garbage: files that are not referenced by any of our customer's rootcaps. The priority of GC work depends upon what this percentage is: if nobody ever deleted a file, then the garbage percentage would be low, and we wouldn't really card about garbage collection. When it is high, we can see how much we'd save on server space if we were to hurry up and implement GC.