Separate `partition` command from command to enable the locality-based CBO

The MovR load generator has a convenience command for partitioning data to improve performance in geo-distributed environments: https://github.com/cockroachdb/movr/blob/master/loadmovr.py#L280

Today this one command does geo-partitioning for the partitionable tables, but *also* adds duplicate indexes for the `promo_codes` table: https://github.com/cockroachdb/movr/blob/master/movr.py#L272

This works fine, but when doing a demo, you may want to show the effects of partitioning, and then run a separate command to show the effects of replicated indexes.

This update will make that demo flow easier. The workaround is to just use `--preview queries` and copy the two command sets separately, but that isn't very elegant.