Lowercase repo name, on creation

[digitalronin]

If the team name in the user's variables.tf file has upper-case letters (e.g.
WebOps), then ECR creation will fail because AWS will reject the name
WebOops/my-test (assuming a namespace called my-test).

This PR helps to mitigate this:
ministryofjustice/cloud-platform-cli#38

However, users can still edit their variables.tf file, and the upper-case team
name is technically correct, so we should adjust this module so that it still
works correctly in that case.