borg-backup: randomise time on a per-server basis

Currently this randomises the minute based on a seed generated from
the backup server name; i.e. all hosts going to a particular backup
server get the same minute.  Use the inventory_hostname of the host
actually being backed up as the seed; this will distribute the backups
over the hour as originally intended.

Change-Id: If25587492e057bed765c91ea759af43293775126

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

@@ -67,7 +67,7 @@
     # 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) }}'
+    minute: '{{ 59|random(seed=inventory_hostname) }}'
   with_inventory_hostnames: borg-backup-server
   loop_control:
     extended: yes