Feature Request: Validate submodule contracts

[phene]

A given Terraform module that you are linting may depend on other modules. We can discover:

1. What variables are we passing in a module block
2. What variables existing in the module and which ones are required (those without "default" values) and what type they are

It would be great to have a lint rule (or multiple rules) that can:

1. Verify that all required variables for a module are provided
2. Verify that no invalid variables are passed to a module
3. Verify the types of the values passed are compatible with the module's specification.

[wata727]

I believe this is the responsibility of terraform validate.