Add support for tm-load-test

We need support for load testing with [tm-load-test](https://github.com/interchainio/tm-load-test), as per the [spec](https://github.com/interchainio/testnets/blob/master/docs/network-layout-spec.md).

A good way to go about doing this would be to simply start with the standalone mode of operation for `tm-load-test`. It's not clear right now whether there's a need/benefit to using the master/slave mode. Perhaps we can consider implementing this at a later stage.