Add statsd attributes

If swift-private-cloud/swift_common/log_statsd_host is set, then the
remaining statds options will be applied to the default section of all
swift-server configs.  Valid statsd options in
swift-private-cloud/swift_common are as follows:

  * log_statsd_host (defaults to nil)
  * log_statsd_port (defaults to 8125)
  * log_statsd_default_sample_rate (defaults to 1.0)
  * log_statsd_sample_rate_factor (defaults to 1.0)
  * log_statsd_metric_prefix (defaults to nil)

Closes #52