Implement dry-run mode at cli level (partially)

This patch adds support for the dry-run mode for all command line
operations, and also makes use of this for commands using the
SubmitOrSend function. For the ones not using it, the flag has no
effect (future patches).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>