[task] allow negative intervals for add_looping_task (closes #2818280)
Instead of rejecting values < 0 with a ValueError, we ignore tasks with
a negative interval. This introduce a simple way to disable task from
configuration.
#! /bin/sh -e
if [ -x "/etc/init.d/cubicweb-ctl" ]; then
invoke-rc.d cubicweb-ctl restart || true
fi
#DEBHELPER#
exit 0