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/defaults/main.yml

@@ -0,0 +1,6 @@
+koji_gc_delay: 1 day
+koji_gc_grace_period: 1 week
+kojihub_host: '{{ ansible_fqdn }}'
+kojihub_url: https://{{ kojihub_host }}/kojihub
+kojiweb_hostname: '{{ kojihub_host }}'
+kojiweb_url: https://{{ kojiweb_hostname }}/koji