digital_ocean.py enhancements

* Always refresh droplet information (versus getting it from cache) for
  --list, --host, --droplets, and --all.   All the DigitalOcean calls are
  pretty fast and without this change, inventory was often stale/incorrect.
  For example, previously if you destroyed a droplet, the inventory
  script would still find it and attempt to act on it.

* Added --force-cache option

* Added some option shortcuts -d, -r

* Added more value sanitization