Add a dry-run option

[retpolanne]

It would be great to have a dry-run option on Teresa, allowing us to understand what exactly Teresa creates on Kubernetes.

We could take advantage of the client https://github.com/luizalabs/teresa/blob/master/pkg/server/k8s/client.go for that by adding a dry-run to every function that creates or updates something on k8s.

The output could be somehow piped back to the client or saved to a bucket.