audit-log: Add controller config for audit logging

[babbageclunk]

Description of change

Plumb the audit logging configuration options into controller config.

QA steps

Bootstrap a controller without auditing enabled (the default) - /var/log/juju/audit.log isn't created. Kill the controller and bootstrap again with config settings:

auditing-enabled=true
audit-log-max-size=100M
audit-log-max-backups=5
audit-log-capture-args=true

See that the audit log file is created and it contains audit log entries.
Check that the options above are shown in the output of juju controller-config.

Documentation changes

These controller config options will need to be documented.

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju