icinga: stop pages from 5xx alerts

As the alerts are all too common and tend to recover quickly, it's better
to remove them for the moment while we refine the threshold and/or anomaly
detection.

Change-Id: I0d0fb1657d79f787e3ab1ed1e1f2ccf20e211a80
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>