Convert to TF 0.12. Add tests. Add Codefresh test pipeline

[aknysh]

what

• Port module to Terraform 0.12
• Pin all providers
• Add example for testing
• Add bats and terratest for the example
• Add Codefresh badge to point to the test pipeline in terraform-modules project
• Update README

why

• Module currently does not work with 0.12. Much easier syntax
• Better regression control
• Automatically test the example on every commit and pull request
• Provision resources on AWS in the test account and check the outputs for the correct values
• terraform-modules project contains pipelines for all terraform modules

related

• Closes upgrade 0.12 #15
• Closes Upgrade to terraform 0.12 #16
• Closes Add Example Usage #11

notes

• terratest for the example will be added in the consecutive PR after https://github.com/cloudposse/terraform-aws-dynamodb is converted to TF 0.12 since both modules depend on each other