Vagrantfile,util/packer: Move Virtualbox DNS fix to build

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
util/packer: Check certificate when downloading vagrant insecure key

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #608 from flynn/packer-dns-fix

Move Virtualbox DNS fix to box build script