WSL default name requires `%s`

[supakeen]

The WSL default_name property in the YAML definitions requires the %s to be present. We should change this to use Go templates (like we do in other places nowadays) so it isn't a problem to omit the string format.

Uncovered in: #2482

ValueError: invalid interpolation syntax in 'Fedora-ELN%!(EXTRA string=11)' at position 10

[supakeen]

This is probably a bit larger in scope, I'd like to apply the same idiom we use for installer customizations in the definitions to the image configuration, so with an ExpandTemplates that templates all relevant fields.