LOGIN-279 Create CloudFormation stack for metadata-proxy app

This merge request contains a working stack configuration, but deploys the app at $tier.metadata-proxy.login.middleware.cloud.vt.edu instead of the specified $tier.md.login.vt.edu. The latter is a CNAME to the former that is not yet created in VT (not Route53) DNS. Once the CNAME records for the tiers exist in VT DNS, the Hostname property in the project can be updated to use it instead. The .gitlab-ci.yml file remains configured to deploy on the branch.