I've stumbled upon the same issue with bootstrapping 7.0 in AWS. We have infrastructure as a code and every single AWS resource is managed from CloudFormation. And manipulating of ec2 instances or even ASGs outside of CloudFormation to bootstrap a cluster seems to me even more error-prone process.

I guess, with `discovery.zen.minimum_master_nodes` parameter removed there is no other reliable way for discovery-ec2 plugin to determine the initial number of master eligible nodes. As @DavidTurner mentioned the AWS API might be flaky.

However, the functionality that used to be in the plugin will still need to be workarounded in UserData.