Having E2E tests for prometheus/test-infra

[geekodour]

After having the KIND provider added #390 added, it'll be possible to have E2E tests for prometheus/test-infra so that we don't have to manually test every change. We can have some sort of /test-infra command to test infra itself.

It's still possible to run E2E tests on GKE, but with KIND this will become more easier and one could just run it locally (need to think of how we can generate test docker images). Just creating this issue to discuss things around it.