logging/agent.pp: provide greater flexibility for sources

Using `$sources` as parameters one can specify the sources
per host/role for fluentd logging.

```yaml
---
  sources:
    apache:
      configfile: apache
      type: tail
      format: apache
      tag: system.apache
      config:
        path: /var/log/apache
        pos_file: /var/db/td-agent/td-agent.apache.pos
    syslog:
      configfile: syslog
      type: tail
      format: syslog
      tag: system.syslog
      config:
        path: /var/log/syslog
        pos_file: /var/db/td-agent/td-agent.syslog.pos
```