Allow a custom release to be used with Test::Instance::Staging and related scenarios

The following discussion from !252 should be addressed:

• @mlapierre started a discussion: (+1 comment)

  ... I think we should also be able to specify the image to use, so we can override the default behavior if required.

Specifically, the staging scenario (and preprod and production) should be able to use a custom release specified via the command line, as it works for Test::Instance::Image.