builder/digitalocean: send a "shutdown" before snapshotting

Instead of pulling the plug on a droplet with the "poweroff" command,
we first send a soft shutdown to the API, then we poweroff to allow
the snapshot to properly complete.

Sending just a shutdown and then snapshotting wasn't as reliable as
sending the poweroff manually, for reasons unknown to me.

This fixes #332.
Merge pull request #339 from mitchellh/do-soft-shutdown

builder/digitalocean: send a "shutdown" before snapshotting