CLI: remove command opts/args in "gnt-X"

[Forward-port of the 1.2 branch patch]

This patch removes all the parameters and options from the output
"gnt-X" (i.e. the subcommand list for command). This is done in order to
uniformize the output, currently only some parameters are shown and they
are not always consistent (e.g. required versus important parameters).

Reviewed-by: ultrotter