Fix proxy_test logging crash when no IPv6 available

Summary:
The test tried to use the self.log member in `__init__()`
but it is only set up in main() in BitcoinTestFramework.

This moves the calls into setup_nodes() which is called
by main, and should log correctly if IPv6 is not available.

Test Plan:
  qa/pull-tester/rpc-tests.py proxy_test
check that the test no longer fails on Travis

Reviewers: #bitcoin_abc, deadalnix, CCulianu

Reviewed By: #bitcoin_abc, deadalnix, CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D504

qa/rpc-tests/proxy_test.py

@@ -49,6 +49,7 @@ def __init__(self):
         self.num_nodes = 4
         self.setup_clean_chain = False
 
+    def setup_nodes(self):
         self.have_ipv6 = test_ipv6_local()
         # Create two proxies on different ports
         # ... one unauthenticated
@@ -81,7 +82,6 @@ def __init__(self):
             self.serv3 = Socks5Server(self.conf3)
             self.serv3.start()
 
-    def setup_nodes(self):
         # Note: proxies are not used to connect to local nodes
         # this is because the proxy to use is based on CService.GetNetwork(),
         # which return NET_UNROUTABLE for localhost