[Security] Add secure install and slow query log support

[tousefanjrmc-ci]

This PR enhances MySQL security hardening and introduces slow query log auditing for the role.

Security hardening items:

• Disable remote root login (allow only localhost/127.0.0.1/::1).
• Remove anonymous MySQL users.
• Remove the default test database.
• Flush privileges after changes.
• New variable mysql_secure_installation (default: true) to gate execution of secure-install.yml.

Slow query log configuration logic:

• New include task slow-query-log.yml is conditionally executed when mysql_slow_query_log_enabled is true (default: false).
• Generates /etc/mysql/conf.d/slow-query.cnf with:
  • slow_query_log = ON/OFF based on mysql_slow_query_log_enabled.
  • long_query_time = mysql_slow_query_time (defaults to numeric 2 seconds if not defined).
  • slow_query_log_file = /var/log/mysql/slow.log.
• Ensures /var/log/mysql directory exists and is owned by mysql with the correct group.

Compatibility notes:

• Slow query log auditing is disabled by default; existing users are unaffected unless they enable mysql_slow_query_log_enabled.
• Secure installation tasks are controlled via mysql_secure_installation (default: true) and are idempotent, ensuring baseline security without disrupting compliant systems.

Implementation details:

• defaults/main.yml: added mysql_secure_installation (true) and set mysql_slow_query_time to numeric 2.
• tasks/main.yml: added two conditional includes after Configure MySQL, before databases/users/replication tasks.
• tasks/secure-install.yml: mysql_secure_installation equivalent tasks.
• tasks/slow-query-log.yml: slow query log configuration and directory setup.

All new/updated YAML files pass basic yamllint (indentation and spacing).

[tousefanjrmc-ci]

安全加固：在此 PR 中通过 secure-install.yml 执行删除匿名用户、禁用远程 root 登录、删除 test 数据库，并刷新权限，确保基础安全合规。

[tousefanjrmc-ci]

慢查询日志：新增 slow-query-log.yml，动态生成 /etc/mysql/conf.d/slow-query.cnf，默认 long_query_time 为 2 秒，可通过变量覆盖，并确保日志目录和权限正确。

[tousefanjrmc-ci]

安全加固：已实现禁用远程 root 登录、删除匿名用户、移除 test 数据库等基础安全加固措施。

[tousefanjrmc-ci]

慢查询日志：新增 slow-query-log.yml，根据变量生成 /etc/mysql/conf.d/slow-query.cnf，默认 long_query_time 为 2 秒，可通过 mysql_slow_query_time 覆盖。