Make EC2 AMIs public

[bboreham]

When I last built them, all the AMIs ended up "private".

The command-line way to make one public is:

aws ec2 modify-image-attribute --region=us-east-2 --image-id ami-6a0b350f --launch-permission "{\"Add\": [{\"Group\":\"all\"}]}"

but I don't see a way to do all eleven of them at once.

Related: #142, https://github.com/weaveworks/scope/pull/3339