fix missing attributes on object when not provided via ini file or comma...

...nd line

This fixes the following traceback:

```bash
bird:ansible-digitalocean benjixx$ ./digitalocean_inventory.py --client-id xxxx --api-key xxxx
Traceback (most recent call last):
  File "./digitalocean_inventory.py", line 307, in <module>
    DigitalOceanInventory()
  File "./digitalocean_inventory.py", line 88, in __init__
    self.cache_path = self.cache_path or '.'
AttributeError: 'DigitalOceanInventory' object has no attribute 'cache_path'
```