Scaleway: add user-data to builder configuration

[ghost]

This issue was originally opened by @mvaude as hashicorp/packer#6776. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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• Packer version: 1.3.1
• builder: scaleway

cloud-init and other kind of user-data could be given in the builder configuration.
As discussed with @sieben, we could migrate to scaleway SDK from @nicolai86 for this kind of features.

@dimtion @edouardb @fflorens @nicolai86 @sieben