python-common/cryptotools: stop using the removed X509Req API

pyOpenSSL deprecated OpenSSL.crypto.X509Req in 24.2.0 (2024-07-20) and
removed it in 26.3.0 (2026-06-12). as we don't pin pyopenssl, CI picked
up the new release, and create_self_signed_cert() started failing with:

  AttributeError: module 'OpenSSL.crypto' has no attribute 'X509Req'

this took down run-tox-mgr, run-tox-mgr-dashboard-py3 and the mypy check.

we only used X509Req to build a subject name and then copied it into the
X509 cert. so drop it, and set the subject on the cert directly. the
resulting cert stays the same: subject from dname, issuer set to the same
subject, self-signed.

Fixes: https://tracker.ceph.com/issues/77391
Signed-off-by: Kefu Chai <k.chai@proxmox.com>