verification: subunit2json fail to open result file

Command "rally verify start" gives the following error:

TRACE rally   File "/usr/lib64/python2.7/site-packages/rally/verification/verifiers/tempest/subunit2json.py", line 113, in stopTestRun
TRACE rally     with open(self.results_file, 'wb') as results_file:
TRACE rally TypeError: coercing to Unicode: need string or buffer, int found

Python open() requires a string but not file discriptor number.

Signed-off-by: Vu Tran <vu.tran@windriver.com>

diff --git a/rally/verification/verifiers/tempest/subunit2json.py b/rally/verification/verifiers/tempest/subunit2json.py
index 719f814..f74b13c 100644
--- a/rally/verification/verifiers/tempest/subunit2json.py
+++ b/rally/verification/verifiers/tempest/subunit2json.py
@@ -137,7 +137,7 @@ class FileAccumulator(testtools.StreamResult):
 
 
 def main(subunit_log_file):
-    results_file, _ = tempfile.mkstemp()
+    _, results_file = tempfile.mkstemp()
     result = JsonOutput(results_file)
     stream = open(subunit_log_file, 'rb')
 
@@ -161,8 +161,8 @@ def main(subunit_log_file):
         suite = subunit.ProtocolTestCase(bytes_io)
         suite.run(result)
     result.stopTestRun()
-    with open(results_file, 'rb') as results_file:
-        data = results_file.read()
+    with open(results_file, 'rb') as results_file_fd:
+        data = results_file_fd.read()
     try:
         os.unlink(results_file)
     except OSError as e: