Add Printer Columns for Gatling CR

[yokawasa]

Feature request

From k8s 1.11, kubectl get can ask the server what columns to display. It would be great if some useful Gatling status will be displayed with kubectl get gatling <name>

relevant pages:

• https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#additional-printer-columns
• https://book.kubebuilder.io/reference/generating-crd.html#additional-printer-columns

[yokawasa]

I'll take care of this

Self-assigned to @yokawasa

[yokawasa]

Closing the issue as the relevant PR has been merged