Prevent destroy basebox when destroying VM

Sometimes, we need to trash and bootstrap a clean VM (especially when doing some hard tests on the DS). I'm used to `vm:destroy` and `vm:init` again, but the _destroy_ action also vaccum the basebox from the Vagrant cache.

Can we add an option (`-k | --keep`) that'll flush the vm but keep the basebox to prevent an extra re-download of the basebox at next `vm:init`?
