py3: urlfetch: use "file://" prefix with filenames
    
    with py3 urlopen used internally with pyldap doesn't work with raw
    filepaths without specifying "file://" prefix. This works on both
    py2/py3
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>