ipa-lockout: do not fail when default realm cannot be read
    
    When ipa-lockout plugin is started during FreeIPA server installation,
    the default realm may not be available and plugin should then not end
    with failure.
    
    Similarly to other plugins, start in degraded mode in this situation.
    Operation is fully restored during the final services restart.
    
    https://fedorahosted.org/freeipa/ticket/4085