r/wal-g-pg: Schedule daily backups

WAL archives are not much good without a base backup onto which they
can be applied.  Thus, we need to schedule WAL-G to create and upload a
backup periodically.

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

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