icinga: make commons content check critical (paging)
This make the new check added in Ie03fccb3de2e582
(see details there) a critical check which means SMS
are sent out and not just mail and IRC output.
It has been running and stable for about 3 days now
so i'm activating it.
All in response to an outage back in January that had
this as a follow-up.See ticket for more.