Introduce track parameters and allow to override them on the command line

[danielmitterdorfer]

We should provide users an ability to override track parameters on the command line without the need to fiddle with tracks which can be frustrating.

To ensure results are still reproducible, we should also record the parameters somehow in the metrics store (e.g. as meta-data attached to metrics records).

Tasks:

• Add a possibility to override variables on the command line (either directly or by specifying a file of key-value pairs)
• Enhance metrics meta-data to include all track variables

[cdahlqvist]

This is great, as it will make it a lot easier to adjust tracks, e.g. primary shards, #connections, bulk-sizes and durations, to fit environments of different sizes.