roles/koji-gc: Deploy the Koji garbage collector

The *koji-gc* role sets up the Koji garbage collector utility to run
periodically. It uses cron for scheduling. A client certificate is
required for authentication, and must be supplied by placing it in the
`certs/koji/{{ inventory_hostname }}` directory.

roles/koji-gc/files/koji-gc-cron.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+printf 'koji-gc run: %s\n' "$(date)" >> /var/log/koji-gc.log
+
+/usr/sbin/koji-gc --no-mail 2>&1 | tee -a /var/log/koji-gc.log | grep -Ei 'err|fail|warn'
+exit 0