Fix NagiosCfg lense so /etc/nagios/nrpe.cfg is parsed by Nrpe and not NagiosCfg

Currently NagiosCfg lense is in charge of parsing all files under `/etc/nagios/*.cfg`
but` /etc/nagios/commands.cfg`. `/etc/nagios/nrpe.cfg` should also be added as an exception
since it needs to be parsed by the Nrpe lense. This issue currently causes the following
error : `Lenses @NagiosCfg and @Nrpe could be used to load this file`