nagios-nrpe: Rename nrpe.cfg.d to nrpe.d

nrpe.cfg.d is the name currently used in Ubuntu so might as well use that.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
nrpe: Allow nagios servers to connect

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