Update cluster auditing task page

Tidying; also drop details of third-party audit configuration

- aggregated API servers
- remove examples of log shipping with fluentd and Logstash

These examples would find a better home in a blog article or elsewhere
on the web. For example, a new blog article that provides detailed
advice about how to deploy log aggregation, configure log shipping into
the aggregator, and protect the audit logs from tampering.

Author: Tim Bannister