(maint)(packaging) Updates to packaging for improved systemd support and PID dir handling

This PR consists of updates to the packaging to better conform to distro standards around systemd. These updates should support using the included spec files to build packages for OpeSUSE and SLES as well as Fedora/RHEL.

In addition, there is an update to the way we were handling the PID directory (/var/run/puppetdb) to work properly with systemd and ephemeral state directories (/run, /var/run, etc). Previously, the rpm package was creating /var/run/puppetdb at install time for pidfile storage. This will break on modern Fedora/RHEL and SUSE systems as /var/run (which is a symlink to /run) uses tmpfs and will not persist through a reboot.

As the systemd service type is "simple", meaning non-forking, systemd
manages the process directly, and the PIDfile is not needed or used on
those platforms.