Update create-track in rally

[gareth-ellis]

Add option to increase batch size when creating a track to speed up download of data. Update track layout to adhere to best practices

Creating tracks from large corpus can take quite a bit of time, so I have added the option to increase the batch size of scan, so if a user is running with a stable enough network connection and enough hardware resources on the rally instance, they can increase the batch size to speed up download.

I have also updated the track layout to better match how we tend to layout our tracks

[ebadyano]

Thank you so much for the changes! Does it make sense to update command line reference to include the new option to increase the batch size? And update the create -tracks docs?

[gareth-ellis]

Yes, that makes sense. I'll update those too

[ebadyano]

tested with regular index and data streams. I think once we have updated docs it's good to go!

One interesting thing for data-streams scenario: we create a regular index in the new track, it's the same as the original behaviour and out of scope for this pr, but I wonder if for data-streams it makes sense if we create a track that index docs into data streams?..

[inqueue]

++ @ebadyano's comments. The rest LGTM.