ipa-advise: correct handling of plugin namespace iteration
    
    The API object namespace iterators now yield plugin classes themselves
    instead of their names as strings. The method enumerating through available
    plugins needs to be made aware of this change.
    
    https://fedorahosted.org/freeipa/ticket/6044
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>