borg-backup: prune after successful backup

After a successful backup run, perform a prune step to slim down to
keep the last 7 days, the last 4 weeks and the last 12 months of
backups.

Change-Id: I98d60319e333d22b3b48214d3dd9136e255a341f

playbooks/roles/borg-backup/templates/borg-backup.j2

@@ -43,6 +43,13 @@ ${BORG} create                      \
 
 backup_exit=$?
 
+if [ ${backup_exit} -eq 0 ]; then
+    info "Running prune"
+    ${BORG} prune --verbose --list --prefix '{hostname}-' \
+            --show-rc --keep-daily 7 --keep-weekly 4 --keep-monthly 12
+    backup_exit=$?
+fi
+
 if [ ${backup_exit} -eq 0 ]; then
     info "Backup finished successfully"
 else