Add file with GCE image locations, similar to ec2-amis.nix. Will be used by nixops.

(cherry picked from commit 9d810ddcc1938a90090fd60f8924f4e83acbeee2)

nixos/modules/virtualisation/gce-images.nix

···
+let self = {
+  "14.12" = "gs://nixos-cloud-images/nixos-14.12.471.1f09b77-x86_64-linux.raw.tar.gz";
+  "15.09" = "gs://nixos-cloud-images/nixos-15.09.425.7870f20-x86_64-linux.raw.tar.gz";
+  "16.03" = "gs://nixos-cloud-images/nixos-image-16.03.847.8688c17-x86_64-linux.raw.tar.gz";
+  "17.03" = "gs://nixos-cloud-images/nixos-image-17.03.1082.4aab5c5798-x86_64-linux.raw.tar.gz";
+
+  latest = self."17.03";
+}; in self