[rotang] This CL adds in serving the trooper files from the RotaNG service.

Remaining cron created files coming in following changes.

Can be tried out on:

https://rota-ng.appspot.com/legacy/trooper.js , current_trooper.json, current_trooper.txt

Bug: 889221
Change-Id: Ieaa99bb9900feb14bb0e04e58b337dc51986c8c4
Reviewed-on: https://chromium-review.googlesource.com/c/1270318
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Cr-Commit-Position: refs/heads/master@{#18268}