Started looking at nagios on cumulus.

Created a host for cumulus in /etc/nagios3/conf.d/cumulus-host.cfg

Added user erl as having all access rights in /etc/nagios3/cgi.cfg

The nagios commands seem to be in /usr/lib/nagios/plugins

To check nagios configuration:

sudo nagios3 -v /etc/nagios3/nagios.cfg