Preserve the configured log level

The Log handler fixture manipulates the loggers log level instead
of just setting the log level on it's own Handler.

If the ``level`` parameter explicitly set to None,
it will not change the level, configured by tempest.

Note: It is a reported fixtures issue.
https://bugs.launchpad.net/python-fixtures/+bug/1244256

Change-Id: Ibb6e1e6bba290ac6941fde184eacd241fe078521

tempest/test.py

@@ -216,7 +216,8 @@ class BaseTestCase(testtools.TestCase,
             os.environ.get('OS_LOG_CAPTURE') != '0'):
             log_format = '%(asctime)-15s %(message)s'
             self.useFixture(fixtures.LoggerFixture(nuke_handlers=False,
-                                                   format=log_format))
+                                                   format=log_format,
+                                                   level=None))
 
     @classmethod
     def _get_identity_admin_client(cls):