wsgi: do not respawn on missing eventlet hub
    
    Eventlet 'poll' hub is not available on OSX, wsgi.Server workers have no
    chance to start but the error was not caught and they were respawned
    endlessly. This patch makes these workers not to be respawned.
    
    This has the nice side-effect of avoiding the test suite to get stuck in
    test_multiprocessing on OSX, filling syslog.
    
    Change-Id: I565d21f5c6861102e96d6953ace29cde57e8ec38