Better check for GCE VM

we should do what is being done in GoogleCloudPlatform/google-cloud-go:
https://github.com/GoogleCloudPlatform/google-cloud-go/blob/master/compute/metadata/metadata.go#L259-L267

Looks like folks are reusing appliances which end up with
```
$ cat /sys/class/dmi/id/product_name
Google Search Appliance
```

pkg/credentialprovider/gcp/metadata.go

@@ -39,7 +39,6 @@ const (
 	metadataEmail            = metadataUrl + "instance/service-accounts/default/email"
 	storageScopePrefix       = "https://www.googleapis.com/auth/devstorage"
 	cloudPlatformScopePrefix = "https://www.googleapis.com/auth/cloud-platform"
-	googleProductName        = "Google"
 	defaultServiceAccount    = "default/"
 )
 
@@ -121,7 +120,8 @@ func onGCEVM() bool {
 		glog.V(2).Infof("Error while reading product_name: %v", err)
 		return false
 	}
-	return strings.Contains(string(data), googleProductName)
+	name := strings.TrimSpace(string(data))
+	return name == "Google" || name == "Google Compute Engine"
 }
 
 // Enabled implements DockerConfigProvider for all of the Google implementations.