`carti machine add`: better handling of remote bundles

[miltonjonat]

At the moment, carti machine add requires bundles to be installed locally first, which is a little cumbersome.

I'm actually not sure installing is really needed to add it to the config. Could we allow a bundle to be added without installing it? Alternatively, we could have this command automatically call carti install <bundle> underneath.

[zcstarr]

I see the point here, not sure about adding it just right now will probably happen after this initial launch.