[GCE] image_id alias image_name

GCE compute resources fail to list out available images in the compute_resource_vms gce form partial because it expects an image_id (like in aws). Fog's gce module uses image_name instead!

I'm tagging icco@ruby-fog to line up the google module with aws but this will prevent headaches in the mean time (or if he rejects the PR's)