v1.8.4-criteo2: [BUGFIX] Avoid GetDatacenter* methods to flood Consul servers logs

When calling `GetDatacentersByDistance()` or `GetDatacentersMap()`, an
incorrect condition was used to diplay log message, thus flooding
Consul's logs.

Example of message:

```
  [WARN] agent.router: Non-server in server-only area: non_server=myClientNode area=lan
```

This message is only valid for WAN areas, filter to avoid creating
hundreds of logs/s on our clusters, each time someone is calling this
method.

Our logs were flooded by such messages when migrating our Consul servers
from 1.7.7 to 1.8.4.

This will issue fix #8663