r/wal-g-pg: Schedule weekly delete jobs

WAL-G slows down significantly when too many backups are kept.  We need
to periodically clean up old backups to maintain a reasonable level of
performance, and also keep from wasting space with useless old backups.

roles/wal-g-pg/templates/wal-g-delete.timer.j2

@@ -0,0 +1,9 @@
+[Unit]
+Description=Periodic PostgreSQL backup delete with WAL-G
+
+[Timer]
+OnCalendar={{ wal_g_pg_delete_timer_schedule }}
+Persistent=yes
+
+[Install]
+WantedBy=timers.target