Remove implicit Str to DN conversion using *-attr
    
    DNs represented as strings and passed via --setattr or --addattr
    are no longer implicitly converted to DN type. This solves various
    errors associated with this behaviour, see tickets below.
    
    Unit tests added.
    
    https://fedorahosted.org/freeipa/ticket/3348
    https://fedorahosted.org/freeipa/ticket/3349