Bug 1085505 part1: Update tester image to create "artifacts" directory.

Artifacts are stored in the /home/worker/artifacts directory. If this
directory doesn't exist, the task finishes unsuccessfully.
Bug 1085505 part2: Add "create" and "deadline" fields in the template.

These fields are required for b2gtest tasks.
Bug 1085505 part3: Add the "ci-tests" mach command.

The --task-id cli allows you to pass the task id of the correspondent
build task. That's where the b2g image and test file are get from to
run the tests.
Merge pull request #13 from walac/bugz/1085505

Bug 1085505