Better notification script

Changes:
* on `-odown` event, we should re-enable master
* master should be disabled only on `+odown` event, not `+sdown`
* sentinel never invokes script for `convert-to-slave` event (better to remove an un-tested feature)
* disable old master when switching master (when failover process started manually, we only receive switch-master event)
* redirect curl output to /dev/null (it apperas in sentinel logs, and only contains whitespaces and newlines)