use instance_id as string for plugin sections

If `instance_id` is set in the metadata as an integer, cast it as a
string when used to uniquely identify plugin sections.

Change-Id: I3a9335fb3885c5f3191051aaaeeec94f89a6b89a

cloudbaseinit/init.py

@@ -39,7 +39,7 @@ def _get_plugins_section(self, instance_id):
         if not instance_id:
             return self._PLUGINS_CONFIG_SECTION
         else:
-            return instance_id + "/" + self._PLUGINS_CONFIG_SECTION
+            return ("%s/%s" % (instance_id, self._PLUGINS_CONFIG_SECTION))
 
     def _get_plugin_status(self, osutils, instance_id, plugin_name):
         return osutils.get_config_value(plugin_name,