Added scrub_data parameter to destroy_droplet method

From the Fog Project with the same issue:

>Major security issue: the Digital Ocean API has a parameter on the destroy call to securely scrub the root blockdev on VM destroy, preventing future customers from reading the data left on disk by your VM.
>
>This is surely a digitalocean security issue, but they're passing it on to users by making it a parameter - rather shitty of them. This is documented in their API at https://cloud.digitalocean.com/api_access - see "scrub_data".
>
>Fog does not pass this parameter, leaving Fog-destroyed VMs vulnerable to later customers stealing the data contained on them.

Dop has the same issue with not passing scrub_data to the API on destroy_droplet calls.  Fix incoming.