ctrl: Add dbg log to print received data

This is useful to see for instance the returned values from NITB when
asked for active subscribers.

Change-Id: I5c616646da360457930888b54d9019b76f05fe2f

src/osmo_gsm_tester/osmo_ctrl.py

@@ -67,7 +67,9 @@ class OsmoCtrl(log.Origin):
         self.sck.send(data)
 
     def receive(self, length = 1024):
-        return self.sck.recv(length)
+        data = self.sck.recv(length)
+        self.dbg('Receiving', data=data)
+        return data
 
     def do_set(self, var, value, id=0):
         setmsg = "SET %s %s %s" %(id, var, value)