reverse-proxy service is not displayed for region controller

[Anton Troyanov]

If MAAS is deployed only with the region controller role, `reverse-proxy` service is missing in the services list.

It says 6 is running (and the 6th one is a reverse-proxy which is not displayed)

```
Services

 6 running

 regiond
 bind9
 ntp
 proxy
 syslog
```

The issue is this condition:
```
(node.node_type == 2 || node.node_type == 4) && services.reverse_proxy
```

We want it to be shown if node_type is 3 or 4
```
RACK_CONTROLLER = 2
REGION_CONTROLLER = 3
REGION_AND_RACK_CONTROLLER = 4
```

maas/src/maasserver/models/service.py
```
REGION_SERVICES = frozenset(
    {
        "bind9",
        "ntp_region",
        "proxy",
        "regiond",
        "reverse_proxy",
        "syslog_region",
    }
)
```