config: Write ssconf after renaming instance

This fixes a bug where the ssconf_instance_list file was
not updated after an instance rename.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

lib/config.py

@@ -995,6 +995,9 @@ class ConfigWriter:
                                                              inst.name,
                                                              disk.iv_name))
 
+    # Force update of ssconf files
+    self._config_data.cluster.serial_no += 1
+
     self._config_data.instances[inst.name] = inst
     self._WriteConfig()