As already reported in
CASSANDRA-10920, changes to /var/run/cassandra folder are not persistent across reboots and therefore saving pid file to that location fails. This causes trouble when managing the status of the cassandra service.
The linked issue mentions good enough (and working) workaround (compared to writing new service file for systemd).
Trunk now contains files necessary to produce RHEL compatible RPM in the /redhat folder.