Add "cleanup" step in each of the exercises.

[sofusalbertsen]

After running 1-4, I end up with the following deployments:

kubectl get deployment
NAME               DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
customnginx        4         4         4            4           3h
envtest            1         1         1            1           1h
multitool          1         1         1            1           4h
nginx              4         4         4            4           3h
nginx-deployment   1         1         1            1           43m

I clutters the image for the student that there are things laying around which is not part of the current exercise.

[sofusalbertsen]

This has been done :)