configdrive: return an empty network config when filename is empty

Additionally, don't bother checking for a network config if it isn't going to
be processed.

coreos-cloudinit.go

@@ -163,7 +163,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	if ccm != nil {
+	if ccm != nil && flags.convertNetconf != "" {
 		fmt.Printf("Fetching network config from datasource of type %q\n", ds.Type())
 		netconfBytes, err := ds.FetchNetworkConfig(ccm.NetworkConfigPath)
 		if err != nil {

datasource/configdrive/configdrive.go

@@ -42,6 +42,9 @@ func (cd *configDrive) FetchUserdata() ([]byte, error) {
 }
 
 func (cd *configDrive) FetchNetworkConfig(filename string) ([]byte, error) {
+	if filename == "" {
+		return []byte{}, nil
+	}
 	return cd.tryReadFile(path.Join(cd.openstackRoot(), filename))
 }