Fix regression in test suite
    
    32b1743e5fb318b226a602ec8d9a4b6ef2a25c9d introduced a regression in
    test_serverroles.py, caused by ca_find attempting to log into the
    Dogtag REST API.  (ca_find is called by cert_find which is called by
    server_del during cleanup).
    
    Avoid logging into Dogtag in cert_find unless something actually
    needs to be retrieved.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6178
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>