MDN infra terraform refactor

[limed]

This is a code refactor for terraform, PR does a couple of things

• Every folder within this the infra/apps/mdn/mdn-aws/infra folder is now a module
• Added a multi-region module that has us-west-2 and us-east-1 and we can enable it using a flag
• Some of the generic VPC information is now discovered with an info module written by the nubis team (see: https://github.com/nubisproject/nubis-terraform/tree/develop/info). This is based on some information that we publish through our account deployment process
• Added enable flag so we can turn on and off specific components
• Added environment variable, doing this allows us to drive the creation of this infrastructure using a CI system

I know this PR is sort of big but please review and ask me any questions if you have any.

[limed]

Here is a full picture on how I call terraform https://github.com/limed/mozilla-mdn

[escattone]

Thanks for your work on this @limed! I was able to spin up an initial POC MDN instance using the infrastructure it created.

[escattone]

I re-based this branch by @limed on mdn/master to get rid of the merge conflict. The merge conflict was simple to resolve. It was due to the fact that I had added an additional CloudFront behavior (users/*) during the all-hands to fix a login issue.