Rely on Elasticsearch's default for source mode in TSDB track

[pquentin]

In #375 we changed tsdb's source_mode to synthetic because that was the quickest way to fix CI and nightly tests. However, it would be best to:

• rely on Elasticsearch defaults, which sets source mode to synthetic for TSDB indices and stored for the rest
• while still allow setting a specific source mode

I'm not sure how to do that just yet: maybe some advanced Jinja2 is enough, maybe we need a track.py, or maybe it's not possible without a change to Rally itself.

Additionally, it would be nice to not having to copy that logic twice as we had to do in #375.