title: Concept behind declaring variables in each submodule instead of declaring it once in parent and reusing it in terraform
body: <p>I come from development background. I am newbie to terraform. I was looking into the ways to pass parent module variable/locals to submodule. The purpose behind it to declare all the variables in one place.
  For example I want to declare aws region local value into parent module and pass it to network submodule which uses 'vpc' module from aws provider.
  I am interested into both why and how here.</p>