Use YAML anchors and alias to maintain emails lists

There is lot of duplicating emails list in the job definition. Instead
define them at the top of the jobs.yaml file and mark each list of
people with anchor which can then be refered multiple times.  Should
ease maintenance / addition of people.

Added Niklas to the job:

    ContentTranslation-language-stage.wmflabs.org-linux-firefox

Change-Id: I0b9d0d541ae62d5759abb94c1900e07e5d2dfae9