Remove explicit DEBUG_OPT and add it by default

Since >90% of the commands take the “--debug” option, and all should
actually take it (the gnt-job command is currently missing it), it makes
sense to simply remove this and add it by default in cli.py.

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