Enhance watchdog_setup script.

Enhance watchdog_setup script.

shutdownall script generated by watchdog_setup shutdowns in the node
number order i.e.: 0, 1, 2...  This causes PostgreSQL backend shutdown
when pgpool0 node went down and node 1, 2... trigger failover event,
which is not necessary in the whole shutdown sequence.  Shutting down
in the reverse order (...2, 1, 0) should prevent this and shorten the
whole shutdown sequence.

Also this should prevent occasional 018.detach_primary and
028.watchdog_enable_consensus_with_half_votes test timeout (they use
watchdog_setup).