if download-only is set, we could have a "target" parameter set in the option dict, and set "filename" parameter if not already set

[desaintmartin]

Everything is in the title. If you are ready, I can implement the first, the second, or both, and submit a PR.

We are little by little phasing out our own slapos.recipe.download recipe in favor of this one so that we can share development, and this is the two last drawbacks from switching everything.