Allow user to define if the box_url is insecure

Currently, it's possible to download a box from an insecure server using the command:

```
vagrant box add mybox  https://insecureserver.com/mybox.box --insecure
```

But I think set it in a Vagrantfile is a good feature.

```ruby
Vagrant.configure("2") do |config|
  config.vm.box = "mybox"
  config.vm.box_url = "https://insecureserver.com/mybox.box"
  config.vm.box_download_insecure = true
end
```