nrpe: revert to the nagios user/paths, clean up

There's no reason that I can see to modify init scripts and config files
all over the place to make nrpe run as the "icinga" user and from
/etc/icinga/nrpe.d rather than the nagios user & paths that the package
installs. There's also no reason to modify nrpe.cfg when there's
nrpe_local.cfg.

Clean all that up, revert to the stock init script & nrpe.cfg and leave
a comment to remove all that calls when puppet has been run everywhere.
This does *not*: rm -rf /etc/icinga, deluser icinga, delgroup icinga, rm
-rf /home/icinga (previously left), as it would also affect neon and
there's no good way to do this in puppet. Left for laters :)

Change-Id: I0e5b55e497c34806426742f002db0631aa006e67