Fix S3 bucket deletion.
support virtualbox >=4.3.0
Merge branch 'new-vbox' of git://github.com/proger/nixops

support virtualbox >=4.3.0

Fixes #151
Use <nixpkgs/nixos> instead of <nixos>
virtualbox: corrected type coercion
Use shell instead of nix-env for system profile setup

This makes it possible for nixops to activate systems
even if their Nix db is read-only. nix-env always
require write-access to the db, but since we already have
a built system path, we don't have to use nix-env to just
set up the system profile symlinks.