Allow running prombench on EKS

[geekodour]

Currently, prombench runs on GKE. It'll be good to have it run on EKS as well. If we need to migrate away from GKE, that'll be super useful.

provider code live here: https://github.com/prometheus/test-infra/tree/master/pkg/provider

related #333 (about running prombench locally)

[bwplotka]

I believe this is possible, but it's not tested on CI/we don't run it anywhere.

https://github.com/prometheus/test-infra/blob/master/prombench/manifests/cluster_eks.yaml