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 be also 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"