Fix spoofed hostname format for StatsD's Ganglia emitter

When the spoof bit is set, Ganglia expects the hostname field to have the
format 'ip:hostname'. Although the first part is considered the IP, it is
treated as a string, and the example provided in the Ganglia documentation
merely doubles the hostname, which seems nicer than cooking up some fake IP.

References:
 - http://sourceforge.net/apps/trac/ganglia/wiki/gmetric_spoofing
 - https://github.com/igrigorik/gmetric/pull/3

Emit 'hostname:hostname'

Change-Id: Ic050b00b508f0de0554318df95f5d892a4d165d6