Most APIs return "has a pending event" but power off seems to return
"already a pending event". WTF. Also, just wtf at the DigitalOcean API.
I can't wait until they change their english and this breaks. Because
it is definitely going to happen, and it's going to be such a fun time.
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.