[feature] Allow discovery to check against existing services in Consul

All of our services already register in Consul, together with their health check (script), for service discovery.
I'm looking into deploying them with Nomad but run into an issue with the discovery implementation.

It would be great to be able to configure Nomad, in the service key, to not register the service in Consul and check against an existing health check in Consul. Something like:

```
service {
register = false
check {
type = "consul"
serviceId = "mock-test-${NOMAD_ADDR_http}'
}
}

```

I'm sure there are other people already using Consul for service discovery who would end up with registering services twice if they were using the current Nomad integration with Consul...

Start by tracing the Nomad service configuration and its Consul registration and health-check integration. Define how register = false should interact with an existing Consul serviceId, then verify the behavior against the requested service configuration and existing discovery flow.

Written by the indexing model from the issue text.