Merge pull request #2757 from rickard-von-essen/gce_fix

Fixed GCE builder after dependency change.

builder/googlecompute/driver_gce.go

@@ -219,7 +219,7 @@ func (d *driverGCE) RunInstance(c *InstanceConfig) (<-chan error, error) {
 	for k, v := range c.Metadata {
 		metadata = append(metadata, &compute.MetadataItems{
 			Key:   k,
-			Value: v,
+			Value: &v,
 		})
 	}