JCLOUDS-543: hack to not barf on old images that are no longer listed in the api

preface -- I'm sure there is probably a better way to do this and I'm open to suggestions on changes.

the problem: some of the droplets in my account are long-running, and have image_ids that are no longer returned from DO's /images api call. So, when I'm trying to create a new droplet, it fails here with a NPE while iterating over the existing droplets (trying to find ones that might match the group name).