pybind: update osd blacklist tests for nonce

Update the unit tests to reflect a) that the nonce is no longer mandatory,
b) that it must be an integer.

http://tracker.ceph.com/issues/6426 refs #6426

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>