gnt-* use the correct opcode slot to build opcodes

gnt-* scripts were building wrong opcodes for commands which had the
shutdown_timeout slot (due to missing testing after renaming). Fixing.

Also change SHUTDOWN_TIMEOUT_OPT dest field name to "shutdown_timeout":
it was set to "timeout". It would still work that way, but possibly be
confusing.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>