release-ec2: Make replicated AMIs public

There is a bug in AWS engine making copied public AMIs invisible.
This patch implements a work-around for this problem as described at
https://forums.aws.amazon.com/thread.jspa?messageID=454676&#454676



Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>