keepalived: notify script handling is broken or undocumented

[stintel]

@feckert

The notify options in the default keepalived UCI config suggest you can put a path to a script. This is misleading, as they are handled by hotplug-call, which is handled by /etc/hotplug.d/keepalived/01-user, which only calls /etc/keepalived.user. If you configure a path to e.g. /etc/keepalived/notify.sh in UCI, this script is never called. This leads to completely broken failover scenarios that are non-trivial to debug.