Change semantics of --force-multi for reinstall

Currently, we require both --force and --force-multiple for skipping the
confirmation on instance reinstalls. After offline conversations, this
has been deemed to be excessive, and this patch changes the meaning of
--force-multiple to be a “stronger” force, and not require both.

So, to skip the prompts:
- single instance reinstallation requires either --force or
  --force-multiple
- multiple instance reinstallation requires --force-multiple

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