[DPUL-C] Update load balancer to only get the frontend web servers.

[tpendragon]

Background

In pulibrary/dpul-collections#162 we run frontend and indexer boxes. The indexer boxes run a web server, but mostly for health checks - we don't want to forward users to it.

Acceptance Criteria

• The load balancer doesn't forward requests to the indexer.

Technical Tips

We've added a frontend label to the frontend boxes. This means we should be able to change https://github.com/pulibrary/princeton_ansible/blob/main/roles/nginxplus/files/conf/http/dev/dpul-collections-staging.conf#L6 to be server service.consul service=frontend.dpulc-staging-web resolve;