Shut down AlertManager when config is deleted

[bboreham]

Following on from #629 which did ruler but not alertmanager

[bboreham]

And I think there is a Notifier running inside ruler when the rules are cleared out.

[rade]

We are getting a lot of log noise from AlertManager because of this, e.g.

cancelling notify retry for "webhook" due to unrecoverable error: unexpected status code 404 from http://eventmanager.notification.svc.cluster.local./api/notification/webhook/xxxxx/monitor