refactor yaml labels, add overwrite protection

Signed-off-by: Christian Richter <crichter@owncloud.com>

extensions/thumbnails/pkg/config/config.go

@@ -12,14 +12,14 @@ type Config struct {
 
 	Service Service `yaml:"-"`
 
-	Tracing *Tracing `yaml:"tracing"`
-	Log     *Log     `yaml:"log"`
-	Debug   Debug    `yaml:"debug"`
+	Tracing *Tracing `yaml:"tracing,omitempty"`
+	Log     *Log     `yaml:"log,omitempty"`
+	Debug   Debug    `yaml:"debug,omitempty"`
 
-	GRPC GRPC `yaml:"grpc"`
-	HTTP HTTP `yaml:"http"`
+	GRPC GRPC `yaml:"grpc,omitempty"`
+	HTTP HTTP `yaml:"http,omitempty"`
 
-	Thumbnail Thumbnail `yaml:"thumbnail"`
+	Thumbnail Thumbnail `yaml:"thumbnail,omitempty"`
 
 	Context context.Context `yaml:"-"`
 }