Break up Terraform config into multiple files

It's a little easier to reason about related chunks of configuration
rather than one blob of config.
Update secure config to include mail details
Add mailgun domain and DNS settings

The name fields are a bit hacked in the DNS settings as the value coming
back from mailgun includes the full domain name, not just the sub domain
and dnsimple expects.
Merge pull request #1 from beardandcode/feature/mailgun

Feature/mailgun