reduce icinga/nagios apt check frequency to 24h

When there's a package upgrade pending, of any kind, icinga gets quite annoying, mailing:

***** Icinga  *****

Notification Type: PROBLEM

Service: apt
Host: pergamon.softwareheritage.org
Address: 127.0.0.1
State: WARNING

Date/Time: 2016-01-23 18:58:10 +0100

Additional Info: APT WARNING: 46 packages available for upgrade (0 critical updates).

Comment: []

every 30 minutes.

Such a frequency is justified for other critical issues, but it's not for pending apt upgrades.

We should reduce it to something saner, like 24h.

Migrated from T282 (view on Phabricator)

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