letencrypt: Split app initialization

Tests:

- On startup, LE tries to obtain certs during post app initialization.

- When domain is added, LE tried to obtain certificates.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/letsencrypt/__init__.py

@@ -83,6 +83,9 @@ class LetsEncryptApp(app_module.App):
                                        **manifest.backup)
         self.add(backup_restore)
 
+    @staticmethod
+    def post_init():
+        """Perform post initialization operations."""
         domain_added.connect(on_domain_added)
         domain_removed.connect(on_domain_removed)