mgr/dashboard: Secure the Alertmanger receiver endpoint
Currently it is possible send push notification unauthenticated to the dashboard and the push notifications are not verified if they actually are coming from an Alertmanager instance.
To see whats configurable see https://prometheus.io/docs/alerting/configuration/#http_config
Removing the endpoint is not a solution to be considered as ceph orchestrator is configuring every Alertmanager instance to talk to the receiver of the dashboard.
The receiver is at the moment the only part that can handle multiple Altermanger instances.