External workers reap containers

Fixes `max containers reached`

Worker will mistakenly attempt to connect to garden (`gdn`) over the
ethernet interface, get ECONN. Instead, worker should attempt to connect
to garden over loopback; that's where it's listening.

fixes, from <worker/beacon.stdout.log>:
```json
{"timestamp":"1534476254.482047796","source":"worker","message":"worker.sweeper.failed-to-report-containers","log_level":2,"data":{"error":"Get http://api/containers: dial tcp 10.2.0.154:7777: connect: connection refused","session":"3"}}
```

`gdn` binds to loopback per `ss -lntp`:
```
State      Recv-Q Send-Q              Local Address:Port                             Peer Address:Port
LISTEN     0      128                     127.0.0.1:7777                                        *:*                   users:(("gdn",pid=52134,fd=7))
```

cluster/external-worker.yml

@@ -35,6 +35,7 @@ instance_groups:
         worker_key: ((worker_key))
         registration_mode: forward
       garden:
+        address: 127.0.0.1:7777
         forward_address: 127.0.0.1:7777
       baggageclaim:
         forward_address: 127.0.0.1:7788