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
* redirect curl output to /dev/null (it apperas in sentinel logs, and only contains whitespaces and newlines)
Disable old master when switching master

When failover process started manually, we only receive switch-master event.
Merge pull request #1 from tahajahangir/patch-1

Better notification script