[Antithesis] Allow running a single node cluster config

[serathius]

What would you like to be added?

Add a separate config for single node cluster.

Why is this needed?

To reproduce issues on single node cluster

[serathius]

/cc @henrybear327 @nwnt @joshjms

[joshjms]

/assign

[serathius]

@joshjms Can you also expose an option to change cluster size in Github Action?

[joshjms]

Sure