Attach BuildTimeTrend to the builders

[jdm]

http://buildtimetrend.github.io/service/

We have this hooked up to our TravisCI builds right now, but that information is less clearly actionable/meaningful than the build time data for our actual builders. The documentation suggests there are a couple ways to go about this, either using Keen.io or storing XML data locally. Then we also need to run an instance of the client that fetches this data appropriately.

[edunham]

This can be done through Salt changes, requiring admin intervention only to roll those changes out. The buildtimetrend client should run as its own user so we can decide whether it's on its own or sharing a host.

[aneeshusa]

AIUI, they've discontinued the production instance at https://buildtimetrend.herokuapp.com/, so we'll likely also need to run our own server in addition to clients.

[aneeshusa]

We are currently using the Buildbot API to fetch and upload build time data, thanks to work by Alan in servo/servo#19344, servo/servo#19450, and servo/servo#19524. Data is available in S3 and I believe can be viewed in Google Data Studio.