ZKFC add HealthCallbacks by default, this can do quiteElection at least. but we often want to be alerted if there is fail over occurring(such as send email, short messages), especially for prod cluster.
There is a configured fence script. maybe we can put all these logics in the script. but in reasonable, fence script does only one thing: fence
So I added this patch, we can configure customized HM callback method, if there is no configuration, then only HealthCallbacks is added.