Support multiple nodes

[danielfireman]

esperf currently only works correctly when the cluster has one node. This issue is about adding support for multiple nodes.

Our first approach will be to add the node-name to all metrics. Thus, instead of having: cpu_baseline_poisson_1.csv we would have cpu_baseline_node-1_poisson_1.csv. Leaving the actual metric name in the beginning and the experiment id at the end of the file name.

[danielfireman]

Of course, it is time to start adding tests.