Install and configure NRPE

In order to successfully check metrics such as disk space, we need to install and configure NRPE. This commit does that by:

- installing NRPE
- adding it to the auto-loadable classes in Hiera (as we use Hiera as an ENC)
- opening tcp/5666 to allow NRPE traffic to flow
- setting the allowed_hosts to localhost and our Icinga box's SNAT IP
- setting the allowed_commands to check_disk alone, with critical and warning alerts

We set the arguments to `check_disk` in NRPE to ensure that, for security
reasons, arguments cannot be injected on-the-wire between monitoring host and
remote host.