Add statsd percentile thresholds configuration

[t3hk0d3]

Original statsd support percentile configuration:

https://github.com/etsy/statsd/blob/master/exampleConfig.js

  percentThreshold: for time information, calculate the Nth percentile(s)
                    (can be a single value or list of floating-point values)
                    negative values mean to use "top" Nth percentile(s) values
                    [%, default: 90]

However bucky have hardcoded value -

https://github.com/trbs/bucky/blob/master/bucky/statsd.py#L182