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
1 file changed