nat/vty: Don't assume one can magically add IPv4 addresses to lo

Don't assume that one can just bind to a local address that has
not been configured. Remove the unspecific comment as I don't know
to which other tests it is referred to.

This should fix:
======================================================================
ERROR: testBSCreload (__main__.TestVTYNAT)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./vty_test_runner.py", line 655, in testBSCreload
    msc = nat_msc_test(self, ip)
  File "./vty_test_runner.py", line 1102, in nat_msc_test
    msc.bind((ip, 5000))
  File "<string>", line 1, in bind
error: [Errno 99] Cannot assign requested address

----------------------------------------------------------------------
diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index 59fdd2f..11b788b 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -627,8 +627,7 @@
         return (4244, "src/osmo-bsc_nat/osmo-bsc_nat",  "OsmoBSCNAT", "nat")
 
     def testBSCreload(self):
-        # use separate ip to avoid interference with other tests
-        ip = "127.0.0.4"
+        ip = "127.0.0.1"
         self.vty.enable()
         bscs1 = self.vty.command("show bscs-config")
         nat_bsc_reload(self)