Report stdout from test listing on failure

stdout from `testr list-tests' often contains useful information, so
report it to the user on failure.

Change-Id: Idc9d2e16d64fcdfad1de881a3458d13a7de33471
Closes-Bug: #1533655

refstack_client/list_parser.py

@@ -50,6 +50,7 @@ class TestListParser(object):
         (stdout, stderr) = process.communicate()
 
         if process.returncode != 0:
+            self.logger.error(stdout)
             self.logger.error(stderr)
             raise subprocess.CalledProcessError(process.returncode,
                                                 ' '.join(cmd))