adtrustinstance: Move attribute definitions from setup to init method
    
    Majority of the attributes set in the setup method can be set in the
    __init__ method where they are actually defined (and set to None).
    
    This is true for attributes that hold constant values and for
    attributes that have their values derived from api.env dictionary.
    
    Creates a new __setup_default_attributes method, that is called
    from within __init__ and setup (in case the passed values for
    hostname or domain do not correspond to that what is set in api.env,
    doing otherwise could cause unexpected behaviour).
    
    Part of: https://fedorahosted.org/freeipa/ticket/3479