Extract AWS Batch Terraform module into separate repo

[kenoir]

In order that the AWS Batch terraform module is reusable both internally and by others outside Wellcome we should move it into it's own repo and consume it as an external module.

In addition it should be switched to use the new CloudFormation provisioning for AWS Batch.

A (private) repo currently exists here:

https://github.com/wellcometrust/aws_batch_tf

When work begins it should be made public.

[kenoir]

Superseded by hashicorp/terraform-provider-aws#1048