WIP: Issue 7055 - publish gecko builds through CD

With all the delay already postponing this, i decided to give a preview of what i'll merge, once everything is done.

Feel free to start the review, despite this being WIP.

Blockers:

• http://hub.eyeo.com/issues/16345 / eyeo/devops/ansible-role-gitlab-runner!3
• http://hub.eyeo.com/issues/19913

(Note that all of the above are also my work)

Cheers!