Support secret parameters

[cosmo0920]

Fluentd 0.12.13 or later supports secret parameter feature.
This feature works as below:

  <source>
    type cloudstack
    host $cloudtack_host
    apikey xxxxxx
    secretkey xxxxxx
    protocol $cloudstack_protocol_scheme
    path $cloudstack_path
    port $cloudstack_port
    interval $get_interval_sec
    ssl $cloudtack_api_ssl
    domain_id $cloudstack_domain_id
    tag $fluentd_tag
    log_dir $fluentd_log_dir
    debug_mode $boolean
  </source>
</ROOT>

If one use older fluentd, :secret => true will be simply ignored.