Run automated tests with more than one cluster at a time

[pebrc]

We are currently e2e testing typically with only one cluster at a time (exception: remote cluster test). Scale testing has so far been a manual effort (#357 )

While a full blown automated scale test is maybe a lot of work for an unclear return, there is value in testing in automated fashion that ECK can orchestrate more than one cluster at a time. We should therefore:

• design a test case that runs multiple clusters (10/100?) at a time
  • define success criteria/metrics for this test case (no errors, but should we also for example measure reconciliation throughput or memory footprint of the operator?)
  • define test steps (just a simple deploy, deploy + rolling upgrade?)
• implement the test case/job
  • it should be runnable independently from our regular e2e test suite given the additional resource requirements (e.g. via separate go build constraints)
  • it needs a CI job/environment that creates a cluster appropriately sized for the larger test case
  • we need to decide when/how to run it. Could be for example part of a pipeline we run after a release candidate build has finished to test for regressions

[pebrc]

Potential duplicate of #1121