Use leaf instead of collect in balance retrieve from api

apps/contract-migration/scripts/cic_eth/traffic/cmd/traffic.py

@@ -241,9 +241,7 @@ class TrafficProvisioner:
             #callback_queue=queue,
             )
         t = api.balance(account, token.symbol())
-        r = t.get()
-        for c in t.collect():
-            r = c[1]
+        r = t.get_leaf()
         assert t.successful()
         #return r[0]['balance_network'] - r[0]['balance_outgoing']
         return r[0]