[master] Trivial fix to address regression after #5175.

The example config for control agent uses IP address of 127.0.0.1
rather than localhost. This caused regression in some unit tests
that use this configuration. The trivial fix committed on my
discretion.

src/bin/agent/tests/testdata/get_config.json

@@ -22,7 +22,7 @@
                 }
             }
         ],
-        "http-host": "localhost",
+        "http-host": "127.0.0.1",
         "http-port": 8000
     }
 }