Allow running an experiment that already exists

[dmartin35]

When the CRD creates an Chaos Experiment, it cannot be created again, unless manually deleted first.

We could add a flag in the experiment definition, to indicate that the [existing] experiment can be deleted/updated with the new one.

This is mainly useful when we want to run multiple times a same experiment with simply applying it , without explicit deletion
kubectl delete ctk x & kubectl apply -f manifest.yaml

We could use a flag like

spec:
  replaceExisting: true

[dmartin35]

similar to #14