Better VirtualBox detection in Vagrantfile

This patch improves detection of the VirtualBox provider during Vagrant provisioning so that it considers the environment variable `VAGRANT_DEFAULT_PROVIDER`. If it's set, it will now assume that you are not using VirtualBox. The VB guest additions will still be installed in some unusual cases, e.g.:

* `VAGRANT_DEFAULT_PROVIDER=virtualbox vagrant up`
* `VAGRANT_DEFAULT_PROVIDER=vmware_fusion vagrant up --provider virtualbox`

However, users are unlikely to invoke `vagrant` with those combinations as they don't really make sense. This simply makes an educated guess based on the presence of the environment variable.