Add a new CONFIRM_OPT option to cli.py

Today we are not very consistent as to what ‘--force’ represents:
sometimes confirmation, sometimes forcing a possible dangerous option,
etc.

This patch adds a new ‘--yes’ option that should be used for all simple
confirmations of genre “yes, I really want to remove the instance”.

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