[rfe] benchmark to accept a list

[jtaleric]

Today we support benchmark: node-density-cni, I would like to see us support a list of benchmarks like benchmark: [cluster-density-v2,node-density-cni,node-density-cni]

The challenge is workloads that don't run things uniform, so the timestamps might be off. To mitigate this, we could do the comparisons per benchmark using the same metadata fields. I think it will make our config less clunky, but maybe more confusing?