Instance autorepair design

This design describes a tool that will perform automatic repairs on
instances when they are detected to be unhealthy (living on offline or
drained nodes, at the moment). These repairs can be scheduled
automatically or requested as a one-off by a tool or person.

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