post-processor/vagrant: add digitalocean support

First, I upgraded the vagrant-digitalocean plugin to version 0.3.0.

Then, I ran packer with:
```
{
"builders": [{
    "type": "digitalocean",
    "api_key": "insert-key-here",
    "client_id": "insert-key-here",
    "image_id": 345791,
    "name": "example",
    "region_id": 3,
    "size_id": 66
}],
"post-processors": [{
	"type": "vagrant",
	"keep_input_artifact": true,
	"output": "example.box"
}]
}
```
The resulting `example.box` contains the following `Vagrantfile`:
```
Vagrant.configure("2") do |config|
  config.vm.provider :digital_ocean do |digital_ocean|
	digital_ocean.image = "packer-1381025582"
	digital_ocean.region = "San Francisco 1"
  end
end
```
Then, I brought up the box via:
```
export VAGRANT_DEFAULT_PROVIDER=digital_ocean
export DO_API_KEY=insert-key-here
export DO_CLIENT_ID=insert-key-here
export SSL_CERT_FILE=/usr/share/ncat/ca-bundle.crt
vagrant box add example example.box
vagrant up
```
When the `status` field is `ERROR`, the message field is always named `error_message`, and not `message`, so I've included support for both.