Consul Connect service health checks not accessible?

### Nomad version

`Nomad v1.0.2 (4c1d4fc6a5823ebc8c3e748daec7b4fda3f11037)`

### Operating system and Environment details

Ubuntu 18.04

### Issue

When running a service binding a port locally (ex `127.0.0.1:8080`), it seems that Consul health checks cannot access them, and I'm unable to use options like `expose` or `address_mode`.

I would expect this to be a pretty common approach if I understand correctly (to avoid leaking ports that could be accessed outside of Consul Connect). Can the guides/docs add steps for health checks in https://www.nomadproject.io/docs/integrations/consul-connect?

### Reproduction steps

Using the following job, try adding `expose = true` or `address_mode = "driver"` to the check and note the errors.

**With `expose = true`**:

```
❯ nomad job run debug/python_http.hcl
Error submitting job: Unexpected response code: 500 (error in job mutator expose-check: unable to determine local service port for service check app->python-http->python-http-health)
```

This happens even if I pass `port = "8080"` in the `check` configuration.

**With `address_mode = "driver"`**:

The job is deployed, but the task fails with the following log:

```
failed to setup alloc: pre-run hook "group_services" failed: error getting address for check "python-http-health": cannot use address_mode="driver": no driver network exists
```

### Job file (if appropriate)

```hcl
job "python-http" {
datacenters = ["kitchen"]

group "app" {
network {
mode = "bridge"
port "http" {}
}

task "python-http" {
driver = "docker"

config {
image = "python:3"
command = "python3"
args = [
"-m",
"http.server",
"-b",
"127.0.0.1",
"${NOMAD_PORT_http}",
]
}

env {
PYTHONUNBUFFERED = "1"
}

resources {
cpu = 20
memory = 100
}
}

service {
name = "python-http"
port = "http"

check {
type = "http"
name = "python-http-health"
path = "/"
interval = "10s"
timeout = "3s"
# address_mode = "driver"
# expose = "true"
}

connect {
sidecar_service {}
}
}
}
}
```

Start with the Consul Connect integration guide at https://www.nomadproject.io/docs/integrations/consul-connect and review the supplied Python job file and health-check errors. Reproduce the `expose` and `address_mode` cases, then document the steps and configuration needed for health checks on locally bound service ports, including the expected behavior.

Written by the indexing model from the issue text.