Would like support for varying sizes of ClickHouse clusters

We're currently [hard-coding support](https://github.com/oxidecomputer/omicron/blob/3445996b4f880dbaab3becdfd5461cff86a7c995/test-utils/src/dev/clickhouse.rs#L326-L330) for a 2-replica ClickHouse cluster, using 3 ClickHouse Keeper instances  for coordination. It would be nice to make these arrays, so that we can manipulate the cluster size more flexibly.