Changes

===Test-driven Refactoring of Marionette's Python Test Runner===

* Mentor: [https://mozillians.org/en-US/u/maja_zf/ Maja Frydrychowicz]

[https://developer.mozilla.org/en-US/docs/Mozilla/QA/Marionette Marionette's] Python Test Runner is slated to become the canonical harness for running most new automated tests for Firefox, but it needs to be lovingly cleaned up and stabilized first. We've already started this work and we're excited to have you help us continue.

* Making the test results more informative and easy to read on [https://treeherder.mozilla.org Treeherder's] log viewer.

* Making the tests more convenient to run locally with mach.

===Add robust AMI management to the TaskCluster AWS Provisioner===