Do not revert options to defaults in SSSDConfig.get_domain()
    
    There was a faulty check in get_domain() that led to the
    *_provider options being re-added, sometimes after options related
    to them had already been set. If those options had a default
    value, they would be overwritten by the default.
    
    Fixes: https://fedorahosted.org/sssd/ticket/441