watcher: write the instance status to a file

This patch modifies the watcher to keep on-disk a file with the instance
status; this can be used from outside of ganeti to react to instances
being down (when the watcher cannot restart them).

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