add Digital Ocean provider plugin

So... they don't use key_name... they do keys by id...
```
ssh_key_ids: [ id1, id2, id3 ]
```
This means you're going to have to do the following:
```
key_id = compute.ssh_keys.select { |k| k.name == @ssh_keypair }.first.id
```
This will give you a key_id that you can pass with ssh_key_ids to your servers.create