Add support to distribute scenarios

[dgzlopes]

I would like to distribute the execution of some scenarios over X load zones while running another scenario only in one load zone.

So, I suppose, this feature request is about:

• Being able to specify this distribution per scenario, instead of relying only on a global test config.

[na--]

FWIW, this should be relatively straightforward to implement if we have test suites (grafana/k6#1342) and native distributed execution (grafana/k6#140). Geo-istribution per scenario is kind of impossible because of the ways VUs and scenarios work together.