Add a link to provisioner in central README
Add a link to provisioner in central README

Currently new users have no idea where the provisioner can be found, and
need to search for it on their own. Since the directory
`tools/aws_lab_setup/` is far from obvious, we add a link to it from the
central README.

ansible/lightbulb,tima/lightbulb,tima/lightbulb,ansible/lightbulb,tima/lightbulb,ansible/lightbulb,ansible/lightbulb