datasource: Add FetchNetworkConfig

FetchNetworkConfig is currently only used by ConfigDrive to read the
network config file from the disk.

datasource/metadata/metadata.go

@@ -38,6 +38,10 @@ func (ms MetadataService) FetchUserdata() ([]byte, error) {
 	return ms.FetchData(ms.UserdataUrl())
 }
 
+func (ms MetadataService) FetchNetworkConfig(filename string) ([]byte, error) {
+	return nil, nil
+}
+
 func (ms MetadataService) FetchData(url string) ([]byte, error) {
 	if data, err := ms.Client.GetRetry(url); err == nil {
 		return data, err