Fix for JCLOUDS-655

First time committing to jclouds, let me know if there are any style guidelines I missed with this change.
+1 to future proofing the name

Have a look at one of the Adapters in [NovaParserModule.java](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/main/java/org/jclouds/openstack/nova/v2_0/config/NovaParserModule.java). You'll want to do something like that for the Image class. Don't forget to add it to the [provideCustomAdapterBindings()](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/main/java/org/jclouds/openstack/nova/v2_0/config/NovaParserModule.java#L57) method.