Fix download url for cn region and update gpg pubkey link.
Fix download url for cn region and update gpg pubkey link.

aws/amazon-ecs-agent,aws/amazon-ecs-agent,aws/amazon-ecs-init,aws/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-init,aws/amazon-ecs-init