Support active connect, overwrite log

Added options:
  --switch-ip   If set, actively connect to this switch on controller-port
  --log-append  Append to existing log

The default log file generation behavior has changed.  By default, now
over-write the existing log.  To have the old append behavior, use the
--log-append option.

Active switch connections have been tested, but not exhaustively.  It
shold still be considered experimental.  The old behavior (listen for
switch connections) should be unchanged.
diff --git a/src/python/oftest/base_tests.py b/src/python/oftest/base_tests.py
index 3664d9f..734e213 100644
--- a/src/python/oftest/base_tests.py
+++ b/src/python/oftest/base_tests.py
@@ -25,6 +25,7 @@
     def setUp(self):
         logging.info("** START TEST CASE " + str(self))
         self.controller = controller.Controller(
+            switch=config["switch_ip"],
             host=config["controller_host"],
             port=config["controller_port"])
         # clean_shutdown should be set to False to force quit app