Set lock_path in tests

When lock_path is not set, the synchronized decorator doesn't work
properly.  This change is required so that synchronized can throw
an exception when that happens instead of failing silently.

Change-Id: I341a522e59fda3feb8e41379d06ad3c8f97e5597

nova/test.py

@@ -30,6 +30,7 @@ import copy
 import os
 import shutil
 import sys
+import tempfile
 import uuid
 
 import fixtures
@@ -251,6 +252,8 @@ class TestCase(testtools.TestCase):
         CONF.set_override('fatal_exception_format_errors', True)
         CONF.set_override('enabled', True, 'osapi_v3')
         CONF.set_override('force_dhcp_release', False)
+        # This will be cleaned up by the NestedTempfile fixture
+        CONF.set_override('lock_path', tempfile.mkdtemp())
 
     def _restore_obj_registry(self):
         objects_base.NovaObject._obj_classes = self._base_test_obj_backup