Use action objects to check status of stopping instance

After the Greenhost API was changed to better follow the DO spec, it now gives a droplet status of off both when it's stopping and when it's really stopped. This is a problem for us, because we wait for status off before deleting a droplet, but that is not yet possible when the droplet is still stopping.

We can instead use the action object that is returned as response to our power_off request, and poll for that action to be completed.