* Implement Stringer inteface for multistep.StepAction
* scaleway: add pre validate step (check image and snapshot names)
Before, it was possible to create multiple images with the same name,
leading to a confusing and wasteful situation (same for snapshots).
Now, we perform the same kind of checks done by the AWS EC2 builder,
and refuse to proceed if there is an existing image with the same name
(same for snapshots).
As usual, invoking `packer build -force` will bypass the checks.
Closes#9839.
