Command for rendering track JSON without running a race

[michaelbaamonde]

It would be convenient if track authors could access the final JSON rendering of a track without actually running a race. This would be especially useful for complex tracks involving a lot of Jinja templating.

Rally does write the fully rendered track to /tmp and logs the path to it, but a dedicated command (e.g. esrally render-track or something) that simply writes the track's JSON representation to stdout would be a nice addition.