Fix a few commands behaviour with dry-run

These commands use or display the result from the LU, so in case of
dry-run, they will crash or display just 'None'. At least checking that
the result is 'true' (in the boolean sense) will make them work better.

As for gnt-os modify, it didn't pass the 'opts' parameter properly to
SubmitOpCode, so the dry-run option was silently ignored.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>