aws_instance to helper/schema, and fixes for helper/schema

This converts `aws_instance` to the `helper/schema` framework. 

In the process, this also fixed a few issues with both:

* Security groups are now treated as a set, so ordering doesn't matter.

* Can set ConnInfo with `helper/schema`

* Fix issues with computed calculation in diff for `helper/schema`

* Add ability to use a processed form of a configuration for diffs (SHA1 user data example)

* `aws_instance` has real validation