iperf3: implement str() method in iperf3 server

This way it can be logged easily during test runs.

Change-Id: I699f0a9a8d263ac9f02ea9594a28ca9fd8d48db6

src/osmo_gsm_tester/iperf3.py

@@ -76,6 +76,9 @@ class IPerf3Server(log.Origin):
     def port(self):
         return self._port
 
+    def __str__(self):
+        return "%s:%u" %(self.addr(), self.port())
+
     def running(self):
         return not self.process.terminated()