datasource: Add FetchNetworkConfig

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

datasource/url/url.go

@@ -1,6 +1,8 @@
 package url
 
-import "github.com/coreos/coreos-cloudinit/pkg"
+import (
+	"github.com/coreos/coreos-cloudinit/pkg"
+)
 
 type remoteFile struct {
 	url string
@@ -33,6 +35,10 @@ func (f *remoteFile) FetchUserdata() ([]byte, error) {
 	return client.GetRetry(f.url)
 }
 
+func (f *remoteFile) FetchNetworkConfig(filename string) ([]byte, error) {
+	return nil, nil
+}
+
 func (f *remoteFile) Type() string {
 	return "url"
 }