borg-backup: space out cron jobs evenly

To avoid the backup jobs running over the top of each other, space the
cron jobs out evenly through the day for each server.

Change-Id: I07a096ee130e61e9efc89887d627da8ef829854a

playbooks/roles/borg-backup/tasks/main.yaml

@@ -58,9 +58,13 @@
     name: "Run borg backup to {{ item }}"
     job: "/usr/local/bin/borg-backup {{ item }} 2>> /var/log/borg-backup-{{ item }}.log"
     user: root
-    hour: '5'
+    # This should space out the backups so they run in a round-robbin
+    # evenly through the day to each of the different backup servers
+    hour: '{{ ((5 + ((24 / ansible_loop.length) * ansible_loop.index0 )) % 24) | int}}'
     minute: '{{ 59|random(seed=item) }}'
   with_inventory_hostnames: borg-backup-server
+  loop_control:
+    extended: yes
 
 - name: Install logrotate rules
   include_role: