roachprod: support running multiple CRDB clusters on the same roachprod cluster

`roachprod` development started a long time ago, and one of the assumptions in a lot of the lower level functions is that _only one_ cockroachdb cluster is running on a roachprod cluster. That was a reasonable assumption to make at the time, as it was meant to test crdb's beahaviour and we didn't have to worry about multiple clusters.

The reality today is different: we have multiple features that involve two CRDB clusters communicating with one another. It's useful to be able to allow multiple CRDB clusters to run on the same roachprod cluster, as that would allow us to test these features with roachtest and experiment using the roachprod CLI.

As an example, one of the features that currently breaks when you try to use multiple cockroachdb clusters in the same roachprod cluster is certificate management, as it assumes `n1` will be started at some point when starting cockroach.

We have existing PCR roachtests, but they work around these issues by starting the CRDB cluster that n1 is a part of first, and sending certificates inline in the `pgurl`.

Jira issue: CRDB-41494