Modified metricKeys to handle multiple graphite sources.

This wasn't all too hard to change, I mostly changed the
single http get to multiple http gets, one for each data
source of type graphite.

It's still not a good idea to call this on the web
frontend, since the whole thing was working for about
20 - 30 minutes when I clicked it, so I'm not
committing my changes to settings.js or the view itself.
Extracted multi-graphite query function, fixed loadRecursive

The new function in metricKeys generalizes what my loadAll
from the previous commit did, it takes a request and a
callback. The request is executed on each graphite installation
and the callback is executed for each result. This makes
implementing loadAll and loadRecursive almost as simple as
it was.