Bug 1289625 - Use checkout-gecko-and-run in mozharness-gecko task; r=dustin

The first thing we do is switch from the desktop-test image to the
lint image: the mozharness tests don't need anything in the
desktop-test image. This does mean converting from Ubuntu 12.04
to 16.04. I'm fine upgrading to a newer operating system by
4 years :)

Once we're on the lint image, we switch to use checkout-gecko-and-run.
The mozharness-gecko task now does version control just like
eslint-gecko and mozlint-flake8.

MozReview-Commit-ID: 6mzAZ7LSBWl

--HG--
extra : rebase_source : a766e5db61998d1cfbcb792ad09a265f45027f54

taskcluster/ci/legacy/tasks/tests/mozharness-gecko.yml

@@ -6,7 +6,7 @@ $inherits:
       build_name: 'mozharness-tox'
       build_type: 'opt'
 
-docker-image: desktop-test
+docker-image: lint
 task:
   metadata:
     name: '[TC] - Mozharness Tox'
@@ -26,13 +26,14 @@ task:
       level-{{level}}-{{project}}-dotcache: '/home/worker/.cache'
 
     command:
+      - /home/worker/bin/checkout-gecko-and-run
+      - /home/worker/workspace/gecko
       - bash
       - -cx
       - >
-          tc-vcs checkout ./gecko {{base_repository}} {{head_repository}} {{head_rev}} {{head_ref}} &&
-          cd gecko/testing/mozharness &&
-          pip install tox &&
-          tox -e py27-hg3.7
+          cd /home/worker/workspace/gecko/testing/mozharness &&
+          /usr/bin/pip2 install tox &&
+          /home/worker/.local/bin/tox -e py27-hg3.7
   extra:
     extra:
         build_product: '{{build_product}}'