(packaging) (#17555) Use systemd for Fedora >= 17 and EL >= 7

Fedora versions >= 17 and EL versions >= 7 use the systemd service
management system, replacing the traditional SysV init system.

This commit configure the RPM package to install the correct systemd
service management scripts and registers the puppetdb service on those
platforms.
Merge pull request #713 from shrug/bugs/17555

(packaging) (#17555) Use systemd for Fedora >= 17 and EL >= 7