introduce site overlays

site overlays are place in sysconfdir/overlays and take
precedence over distribution overlays with the same name.

Every `wwctl overlay` command changing overlays will create
an site overlay. distribution overlays can't be deleted or
modified with wwctl.

Signed-off-by: Christian Goll <cgoll@suse.com>

internal/app/wwctl/overlay/delete/main.go

@@ -22,8 +22,11 @@ func CobraRunE(cmd *cobra.Command, args []string) error {
 		fileName = args[1]
 	}
 
-	overlayPath = overlay.OverlaySourceDir(overlayName)
+	overlayPath, isSite := overlay.OverlaySourceDir(overlayName)
+	if !isSite {
+		return fmt.Errorf("distribution overlay can't deleted")
 
+	}
 	if overlayPath == "" {
 		return fmt.Errorf("overlay name did not resolve: '%s'", overlayName)
 	}