Introduce instance start/stop no_remember attribute

This will allow stopping or starting an instance without changing the
remembered state. While this seems counter-intuitive at first (it will
create cluster verify errors), it can help in a few corner cases:

- shutting down an entire cluster for maintenance but without having
  to remember state
- doing testing of Ganeti itself

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