Add the ability to install nrpe/plugins from packages on rhel/fedora

Installs from EPEL which adds yum as a requirement.  On Fedora >17 it
does not require epel.  There's a new attribute for packages since the
rhel packages break out every plugin into a package.
Describe to new behavior in the README
Merge pull request #134 from tas50/COOK-1068

Cook 1068: Allow the installation of NRPE using EPEL packages on RHEL / Fedora