Fix munin plugins

the basic path for nagiosstats is wrong/hardcoded.

that binary comes in with the nagios3 package in Debian. set the default
value to the right path under Debian and make it possible to override
the path with an environment variable.
munin plugins: Change user to 'nagios' to lessen security impact

we don't need to run these plugins as root, that's way overkill. Let's
run those under a more limited user with access to nagios control files.
Remove useless inheritance for nagios::munin

Let's transform it into a class inclusion since we're not overriding any
resources.