platform/aws: add bootstrap node and step for joining it

[enxebre]

Breaking down #2802 into smaller PRs.
Fix INST-823 and INST-824
At this point this would require:
terraform import aws_autoscaling_group.masters cluster-aws-masters
terraform import aws_autoscaling_group.workers cluster-aws-workers for the joining step. Just a note that in the future the tradeoffs of simplifying this workflow by using single instances could be re-evaluated

[coreosbot]

Can one of the admins verify this patch?

[alexsomesan]

@enxebre What's the reasoning in favor of using import terraform import aws_autoscaling_group.masters cluster-aws-masters versus referencing the autoscaling group via remote state datasource?

[enxebre]

@alexsomesan good question! to my knowledge data source is read-only mechanism so we can take advantage of it to take action based on data from a previews step but I can't see how to modify the desired state of a pre-exiting resource without importing it. The way to avoid it as far as I can see would be to use independent resources, so single instances as stated in the desc above. Otherwise we could eventually remove the tf dependency for this and just scale the groups via cli/aws-api but this would set a hard dependency on the cli