scylla-monitoring-1.0.0

Adding a new cpu dashboard (#336)

* Adding a new cpu dashboard

Replaces: enhance per server dashboard with useful metrics

Adding a new dashboard that specialized in CPU load
 - Adding a graph with foreground CPU utilization. That is the CPU used by
   request processing, excluding compaction, flushes and other things. The reason for that is that users are usually scared of spikes. Even if we tell them that
   spikes are fine because they are the result of isolatable background processes,
   it is hard to *prove* that without further analysis. This graph will help.

 - time spent in violations: A lot of the latency issues we have, especially in
   higher percentiles come from task quota violations. We have a metric for this
   now and it will help us correlate latency spikes in time

 - Client connections: in the past few months, this is *THE* top metric we
   have been looking at to detect problems. It harms us a lot that it is not
   part of the main dashboard.

In the process of doing the above, I am also doing my best to document the new
graphs. The text will appear in the tooltip in the top left corner of the graph.