Add InstanceInfo, related classes and tests (googleapis#955)

* Add InstanceInfo, related classes and tests

* Move InstanceInfo inner classes outside

Author: mziccard

gcloud-java-compute/src/main/java/com/google/gcloud/compute/Compute.java

@@ -2339,7 +2339,7 @@ Operation deprecate(ImageId image, DeprecationStatus<ImageId> deprecationStatus,
    *
    * @throws ComputeException upon failure
    */
-  Page<Subnetwork> listSubnetworks(String project, SubnetworkListOption... options);
+  Page<Subnetwork> listSubnetworks(String region, SubnetworkListOption... options);
 
   /**
    * Lists subnetworks for all regions.