Flatten droplet actions params to match the current API spec

I think the droplet actions APIs have changed, e.g. for resize: https://developers.digitalocean.com/#resize-a-droplet

E.g. it's now
```
{
  "type": "resize",
  "size": "1gb"
}
```
instead of
```
{
  "type": "resize",
  "params": {
    "size": "1gb"
  }
}
```