User custom user agent to identify production test runs

It's important that when we generate the graphs based on
EventLogging data, we have a foolproof way to filter out
local runs that happened during development.

Change-Id: Ifc65d3b4bfb96946a8e8d59d080a6c5ee6552533
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/509
Merge "User custom user agent to identify production test runs"