fix 'No handlers could be found for logger "swift-proxy"' warning

Don't ask me why this works; it just does.  I found the idea here:

  https://ask.openstack.org/en/question/27166/no-handlers-could-be-found-for-logger-swift-proxy/?answer=27172#post-id-27172

but nothing in the manual explains why log_name should matter for
anything other than what appears in the logs:

  http://docs.openstack.org/icehouse/config-reference/content/proxy-server-configuration.html