googlecloudstorage: fall back to default application credentials

Fall back to default application credentials when all other credentials sources fail

This change allows users with default application credentials
configured (notably when running on google compute instances) to
dispense with explicitly configuring google cloud storage credentials
in rclone's own configuration.

backend/googlecloudstorage/googlecloudstorage.go

@@ -16,6 +16,7 @@ FIXME Patch/Delete/Get isn't working with files with spaces in - giving 404 erro
 */
 
 import (
+	"context"
 	"encoding/base64"
 	"encoding/hex"
 	"fmt"
@@ -45,6 +46,8 @@ import (
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 	"google.golang.org/api/googleapi"
+
+	// NOTE: This API is deprecated
 	storage "google.golang.org/api/storage/v1"
 )
 
@@ -381,7 +384,11 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 	} else {
 		oAuthClient, _, err = oauthutil.NewClient(name, m, storageConfig)
 		if err != nil {
-			return nil, errors.Wrap(err, "failed to configure Google Cloud Storage")
+			ctx := context.Background()
+			oAuthClient, err = google.DefaultClient(ctx, storage.DevstorageFullControlScope)
+			if err != nil {
+				return nil, errors.Wrap(err, "failed to configure Google Cloud Storage")
+			}
 		}
 	}