Update AWS setup documentation

[liquidat]

At lightbulb/tools/aws_lab_setup/README.md fix follwing things:

• mentioning of redhatgov should be dropped in favor of our own solution (when available)
• add information where to get VPC subnet id
• streamline howto, current mentioning of no-email at the beginning can be improved

@tima You can assign me to this directly, I am on those.