added disk space progress bar for each ES cluster node

I wanted a quick overview on the available disk space of each node, so I added this:

![fs-size-bar](https://cloud.githubusercontent.com/assets/938061/2720545/cd2fa4c0-c570-11e3-868a-0028d4ad33e5.png)

I also tried extending the unit tests, but I had to adjust the stats_90 node JSON so that it has the same disk space every time. Otherwise the tests would fail.

Have a look at it, maybe you need to adjust the style/design of the new progress bar.