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/templates/koji-gc.crond.j2

@@ -0,0 +1,7 @@
+{% if koji_gc_notify is defined %}
+MAILTO={{ koji_gc_notify }}
+
+{% endif %}
+40 3 * * * root /usr/local/libexec/koji-gc-cron
+50 3 * * 0 root find {{ koji_home }}/scratch -mindepth 1 -not -newermt '-60 days' -delete
+55 3 * * 0 root find {{ koji_home }}/work -mindepth 1 -not -newermt '-30 days' -delete