gcs: empty directory markers #3453

- Report correct feature flag
- Fix test failures due to that
- don't output the root directory marker
- Don't create the directory marker if it is the bucket or root
- Create directories when uploading files

fstest/test_all/config.yaml

@@ -129,6 +129,9 @@ backends:
  - backend:  "googlecloudstorage"
    remote:   "TestGoogleCloudStorage:"
    fastlist: true
+ - backend:  "googlecloudstorage"
+   remote:   "TestGoogleCloudStorage,directory_markers:"
+   fastlist: true
  - backend:  "googlephotos"
    remote:   "TestGooglePhotos:"
    tests: