cluster-merge: add instance restart strategy opt

Right now we always restart all instances, which is not right if some
instances were already down for other reasons. Thus we add an option to
decide how to handle this. The right default should be "up" which is:
"restart all options which were switched off by the merge", but since
that's not implemented yet, the default remains the old one, for now.

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