Just one dude's opinion, but Nagios Core isn't really the right tool for managing updates/configuration of machines. You *could* do it, but that doesn't mean you *should* do it. Nagios Core is much better suited as a monitoring framework than a configuration management framework.
I would highly recommend one of the tools I previously mentioned in this thread (and
the other thread) for that specific use case. They are, among other things, much easier to setup for this particular situation.