support "disable" on diamond::collector

[neoice]

it looks like diamond ships the following metrics by default: cpu, diskspace, iostat, loadavg, memory, vmstat

I can override their configuration easily:

  # collect CPU as a simple, normalized metric
  diamond::collector { 'CPUCollector':
    options => {
      'normalize' => true,
      'percore'   => false,
      'simple'    => true,
    }   
  }

but I cannot explicitly disable a collector using this resource.

  # disable iostat
  diamond::collector { 'DiskUsageCollector':
    options => {
      enabled => false,
    }
  } 

this obviously fails :P