nagios: Re-organize nagios-nrpe and nagios-plugins

All the existing plugins have been moved to a new
nagios-plugins-stgraber.net package so there's no reason to continue
shipping the scripts in the repository.

This also updates /etc/nagios/nrpe.cfg.d to automatically purge unwanted
entries and to have a working group => filename mapping.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>