Attach the configured elastic IP to rebooted instances

After rebooting an instance (`cap rubber:stop` and `cap rubber:start`) and refreshing the instance (`cap rubber:refresh`), the instance's save file will still have the old value for `static_ip` and a new, different value for `external_ip`.

This pull request will associate the elastic IP that the instance is configured to have during `post_refresh` by checking if the instance's saved external IP and its configured Elastic IP match and associating if they don't.