[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
 
case "$1" in
    purge)
	rm -rf /etc/cubicweb.d/
	rm -rf /var/log/cubicweb/
	rm -rf /var/lib/cubicweb/
    ;;
esac
 
#DEBHELPER#
 
exit 0