enable statsd reporting for swift proxy

ATM only the proxy template has statsd reporting, will submit another code
review to extend that support to other components

also set the default sample rate to 10% as a first attempt to not swamp stats
with traffic. note the metric name, the sample_rate metric used before has been
introducted in swift 1.8 so it can and will be used after we have upgraded
https://bugs.launchpad.net/swift/+bug/1090495

Change-Id: I0bcdc959769b64867d92198b9da42b6b2574731a