Make sure droplet deletion works even when idempotency is achieved using 'name' and 'unique_name' rather than 'id'

So this fixes a bug that makes it impossible to delete a droplet that is specified with just a combination of name and unique_names (for idempotency).

In particular, without the fix, this code: 

```
digital_ocean:
  image_id=1646467
  name=mydroplet
  ssh_key_ids=54036
  size_id=63
  region_id=4
  state=absent
  unique_name=yes
```

will always fail with:

```
TASK: [deploy droplets] *******************************************************
failed: [127.0.0.1] => (item=mydroplet) => {"failed": true, "item": "mydroplet"}
msg: Unable to load id

FATAL: all hosts have already failed -- aborting
```

because the module will fail, not finding 'id' (even though, according to the manual and to what I need to do, seems like a completely legit case)