Masked systemd units shouldn't alert
We sometimes have masked systemd units with a failed status, for instance:
* mariadb.service
Loaded: masked (/dev/null; bad)
Active: failed (Result: exit-code) since Sat 2018-10-27 07:00:49 GMT; 10min ago
Main PID: 7939 (code=exited, status=1/FAILURE)
Status: "MariaDB server is down"
CPU: 214ms
this currently results in a SystemdUnitFailed alert. The alert should exclude masked units.