Adds support for gentoo/sabayon, corrects minor flaws

Good job on this module. I needed to make a few small changes to work on my system, and am raising this request in case they are useful to others. This should apply cleanly to master, with no change to the public api of the module.

The three commits in this pull request ensure that:
* the value of nrpe::config::include_dir is used in the generated nrpe.cfg
* the File resources created use the actual destination file path as the $title, rather than the redhat path which lead to some confusion during installation for me
* that this module runs on gentoo/sabayon, and plays nicely with upstream nagios/nrpe packages.

I'm using the following nrpe::config resource to support this:
    nrpe::config {
        'nrpe.cfg':
            pid_file        => '/run/nrpe.pid',
            nrpe_user       => 'nagios',
            nrpe_group      => 'nagios',
            include_dir     => $nrpe::params::confd;
    }