Use concat to allow other modules to contribute to audit.rules

1. Primarily this commit introduces puppetlabs/concat so that external modules may use a newly defined type `auditd::rule` with an appropriate order (default=10) to register new rules in `/etc/audit/audit.rules`.

2. Some cleanup work will now be optional to remove any GOV.UK rules which live in the (newly renamed) `audit.rules.begin.fragment.erb`.

3. Config immutable option is also available (and written as part of the `audit.rules.end.fragment.erb`) and has order 99 to ensure it gets defined as the last rule.

4. Failure mode now available to kill machine should a rule be fired.