User Variables in Intro

The intro was using inline secrets. At the time user variables didn't exist.

User variables are now a first class feature and important thing to teach newcomers – doing so up front is a good thing.

It does add a bit of verbosity to the command line examples (passing in 4 vars at the end) but c'est la vie.

Additionally, the IAM policy was in the chroot builder, I moved it out to common. Though supposedly it doesn't work (see #577).