+ statsd: add hostname-override to statsd config options.

@ivantopo I think something like this would be the idea for the hostname-override. What do you think? 
I think it would be better to always have the key available via our reference.conf file with a default value of `none` and then in code, decide to override or not based on that sentinel value.. That way the effect of this config setting is better exposed by the comments on the reference.conf file rather than people having to dig into the code to know if it is possible or asking on the mailing list. The reference.conf files are a magnet for curious users!