rgw/bucket_notification: enhance Kafka security testing

rgw/bucket_notification: enhance Kafka security testing

Add support for SCRAM-SHA-512, dynamic hostname/IP configuration for SSL
certificates, and improved SASL mechanism handling in Kafka security tests.

Changes:
- kafka-security.sh: Support KAFKA_CERT_HOSTNAME and KAFKA_CERT_IP environment
  variables for dynamic certificate generation with proper SAN fields
- test_bn.py: Add SCRAM user setup, support SASL_SSL/SASL_PLAINTEXT protocols,
  add SCRAM-SHA-512 test cases
- README.rst: Update with step-by-step Kafka configuration instructions
- requirements.txt: Replace nose-py3 with pynose, add requests>=2.28.0

Signed-off-by: sujay-d07 <sujaydongre07@gmail.com>