SSSDConfig: Return correct types in python3
    
    In Python 3, dict.keys() returns a view rather than a list. Since dict keys
    aren't in any particular order, indexing them doesn't make sense.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2699
    
    Reviewed-by: Christian Heimes <cheimes@redhat.com>