naggen2: only pick up resources older than 1 hour by default

This is another possible/partial solution to the "we get spammed
by icinga when a host is first being brought up", the other
being https://gerrit.wikimedia.org/r/#/c/144839 (which is better
in that it uses downtime instead of preventing adding to icinga
at all, but haven't found a good way to trigger it for the right
hosts at the right time).

Change-Id: Ied778ec8f73b06da0a62fa6dd0fca123e250fe87