ceph_argparse.py, cephtool/test.sh: fix blacklist with no nonce

It's legal to give a CephEntityAddr to osd blacklist with no nonce,
so allow it in the valid() method; also add validation of any nonce
given that it's a long >= 0.

Also fix comment on CephEntityAddr type description in MonCommands.h,
and add tests for invalid nonces (while fixing the existing tests to remove
the () around expect_false args).

Fixes: #6425
Signed-off-by: Dan Mick <dan.mick@inktank.com>