This change introduces a new nullStep type that can be used in place of
a nil step. This fixes an issue where multistep.If would return a nil
step if the condition was not met causing a builder to crash when
executed using `-on-error=ask` or `-on-error=abort`.
Closes#10241
* 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.
Wilken Rivera
Marco Molteni
Adrien Delorme
Matthew Hooker