icinga: make snmptt cronjob less spammy

Redirect "service snmptt restart" stdout/stderr to /dev/null to avoid
cronspam.

(I have no idea why would this be needed in the first place and why the
ensure => running Service wouldn't do it, but not willing to debug this
now)

Change-Id: I6788c0c56a6c2232fac564f5033eb16781bd84e2