Add ability to benchmark an already set up cluster

[danielmitterdorfer]

Sometimes we want to benchmark an already setup cluster that is not provisioned by Rally.

We should add a new pipeline that allows to run a benchmark against an already running ES instance with esrally --pipeline=benchmark-only. For now, we will not allow to configure host and port and assume that it is running on localhost:9200.

Note that we will lose the ability to gather lots of metrics like CPU usage and so on. But as this is a very special benchmarking mode which is discouraged anyway for general usage this should be ok.

My main motivation to add this now is to get more insights into how Rally is behaving w.r.t. coordinated omission (see also #9).

/cc @nik9000: you wanted something like this a few weeks ago and I'm about to push this to master. If you have a cluster with a prepopulated index, you'd still have to write your own benchmark (which is quite easy!) and I am happy to help. Just ping me.