Downloader should be more quiet

Would be nice to keep a progress bar but silence the
info. But meh.
Platform image can be downloaded as a registered action

Unclear why env is a hash in middleware, but a Vagrant::Environment
in a command.
Register virtualbox_platform_iso before VB customizes config

Runs twice, but doesn't seem to be a problem. Once before
anything boots, which is desired. A second time somewhere
after VM is booted, after checking for guest additions.