Add missing server attribute in Image

Otherwise retrieving an image based on a server causes the following exception: 

Caused by: org.codehaus.jackson.map.exc.UnrecognizedPropertyException: Unrecognized field "server" (Class com.woorea.openstack.nova.model.Image), not marked as ignorable
at [Source: org.glassfish.jersey.message.internal.EntityInputStream@404f4560; line: 1, column: 691] (through reference chain: com.woorea.openstack.nova.model.Images["images"]->com.woorea.openstack.nova.model.Image["server"])