Set a tempest lock_path default to a tmpdir

This commit sets a default value for the tempest lock_path to be just
be tempset-lock in the tempdir. Unlike the servers which could have
security implications for where the lock path is put, for tempest this
isn't a concern since it has to be manually invoked and has a finite
run time.

Change-Id: I2b326731984be8c9fd2491b52ffb1dfbc379f917
diff --git a/tempest/config.py b/tempest/config.py
index a6212fb..ad2f662 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -17,9 +17,10 @@
 
 import logging as std_logging
 import os
+import tempfile
 
+from oslo_concurrency import lockutils
 from oslo_config import cfg
-
 from oslo_log import log as logging
 
 from tempest.test_discover import plugins
@@ -1462,6 +1463,8 @@
     def __getattr__(self, attr):
         if not self._config:
             self._fix_log_levels()
+            lock_dir = os.path.join(tempfile.gettempdir(), 'tempest-lock')
+            lockutils.set_defaults(lock_dir)
             self._config = TempestConfigPrivate(config_path=self._path)
 
         return getattr(self._config, attr)