[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.

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s repository side views, usually for notification"""