Fix --no-headers for the new list-drbd command

Sorry, I forgot that with GenerateTable this needs to be handled
manually. Fixed now and tested in both ways.

(But to be honest, this should be abstracted in GenerateTable, instead
of the 'if' test in all its callers.)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>